The one consistent thing in this and your other Q's with your sons behaviors are your expectations. They aren't age appropriate.
You're assigning adult motives to very small children, and young children. And adult consequences.
While, yes, most 6 & 8yos know to ask (and do it MOST of the time), it's a 10,000 step process of repetition over years and years. Not something you tell a 3yo once and then punish them for the next 5 years with spankings and their things being taken and being sent away for the weekend. OF COURSE they're going to sneak stuff. They would, anyway, as kids push boundaries... But when they've been taught they're bad and unwanted out of years of unrealistic expectations that is going to take a WHILE (6mo-2years) of age appropriate expectations to undo.
Here's a real-world textbook example one of my psych profs uses in her developmental psych class every year:
Dad and daughter were seeing her for regular counseling (court ordered ). One afternoon dad come dragging in his 6yo by the arm half shouting "She's trying to make me lose my job!!!"
No.
She's 6. What she WANTED was dad to stay home and play with her, so was begging, then moving slow, then throwing a tantrum, then huge fight between dad and daughter.
His expectations were off.
Should he have stated home from work and played? NO.
Was she 'trying to make dad be late so he'd lose his job'? Nope.
Totally normal 6yo want & behavior trying to get that want. But dad was placing adult motivations and reasonings behind a totally normal kid thing to do.
Dad tried to 'prove' he was right by asking 6yo if she wanted dad to lose his job, and she said yes. Why??? Because she's 6. To her mind, a job is where daddy goes when he's not with her, lose the job, he'd be with her. But can she make the logical reasoning to say that If I throw a tantrum, it will make addy lose his job so hell stay home"? Nope. She just had a want. She wanted dad to stay home. Then she had a second want; When dad gave her a great choice just then in the office, she absolutely agreed... Totally unable to link what would happen financially and career climbing if dad list his job.
Because she's a child.
Infants have needs.
Children have needs and wants
Adults have needs, wants, and AGENDAS.
Your kids have wants. Oooooh iPod. I'd like to ______. Pick up the iPod, go do ____ with it. Heck... even with teens and ADULTS you'll often see an item in someone's hand as they shout over their shoulder's "Hey, so-and-so can I use your....?"
And either a want or a need "Mmmm. Yogurt. Hungry." or "Mmmm. Yogurt. Yum."
Assigning an agenda to it "stealing" is apples and oranges. Or heck, apples and footballs.
IMPULSE CONTROL is what's lacking here... on their part.
Again. TOTALLY AGE APPROPRIATE for there to be some poor impulse control in elementary aged kids. AKA knowing better, and doing it anyway. While expectations are off on your part.
Along with the impulse control, are also some learned "asking before using something that doesn't belong to you" which takes ages and ages and ages. ESPECIALLY because you're not talking stealing from kids at school or neighbors and hiding it... but using something in their own home without permission. (NORMAL). Both kids and adults, if it's in the space they live in or work in tend to assign ownership to everything in it. EX) The copier at work, the towels at a hotel, heck, even the parking spots on the public street. If it's always around, there's an automatic possiveness about things that both kids and adults have. AKA iPod is in their home, therefore it's partially theirs. ((Only because they're older. As toddlers it WOULD have been "theirs" in their minds. See below for Toddler Property Laws))
It takes years and years of repetition and age appropriate expectations and guiding for there to be even the pause before using something that belongs to someone else. Just for Fun... here's that ownership thing I was mentioning earlier at an earlier milestone.
Property Law As Viewed By A Toddler
1. If I like it, it's mine.
2. If it's in my hand, it's mine.
3. If I can take it from you, it's mine.
4. If I had it a little while ago, it's mine.
5. If it's mine, it must never appear to be yours in any way.
6. If I'm doing or building something, all the pieces are mine.
7. If it looks like it's mine, it's mine.
8. If I saw it first, it's mine.
9. If I can see it, it's mine.
10. If I think it's mine, it's mine.
11. If I want it, it's mine.
12. If I "need it, it's mine
(yes, I know the difference between "want" and "need"!).
13. If I say it's mine, it's mine.
14. If you don't stop me from playing with it, it's mine.
15. If you tell me I can play with it, it's mine.
16. If it will upset me too much when you take it away from me, it's mine.
17. If I (think I) can play with it better than you can, it's mine.
18. If I play with it long enough, it's mine.
19. If you are playing with something and you put it down, it's mine.
20. If it's broken, it's yours (no wait, all the pieces are mine).
Your kids have moved beyond "mine" into knowing it's not theirs and wanting to use it. That's developmentally normal. The polite way to handle that, and the impulse control surrounding stopping and doing the polite thing? That takes awhile.
I'd really, really recommend family counseling with a developmental psychologist for you and your sons. (psych branches are as different as medical branches... you don't want a dermatologist doing heart surgery). Specifically a developmental psychologist to help you recraft your expectations and reshape the way you think about and act towards your kids when they're doing something totally normal that needs correcting.