Well, would she do it if she had the money? Maybe, maybe not... you are judging her just as she is judging you. Is it like an all day thing, 5 days a week? If so, then yes, I DO agree with her. If it's just a little Mother's Day Out program for a few hours/ a few days a week, then I agree with you, the socialization and play time could be good for him, and it would also be good for you, but I also think that just turning 2, is still very, very young for that. So, it just depends, I don't know the whole story, but that's not the point... I don't think her being jealous has anything to do with it, and I think it's a cop out that people use.
As for "why" women do this? Because sometimes we see people make very stupid mistakes. Sometimes, we don't know the whole story, (or we do). Sometimes we have a differing opinion based on our own personal backgrounds/education/experience...
Also, you gave her the information and she commented back. It's not like she was prying or snooping in order to judge you.