Actually, it's the value of Microsoft Accounts that make them prime targets. People put serious effort into stealing accounts, they check them for value across the account, then they sell them off to others who do what they want with them. Most of the "bad people" concentrate their efforts on Live. Look at the EA Fifa fiasco. Someone gets a hold of your PSN account and it's not that big of a deal. They get a hold of a high value Live account and it can cause major headaches. I've actually been trained as an Xbox support agent, the amount of time and effort they spend on preparing you against spoofing is unreal, and yet it still happens. One of the first ghost calls I sat in on was a guy trying to get into an account "for his brother" who had "gone to the store but asked him to call support".
That makes sense in terms of identity and account theft. However in terms of cheating, team killing and other horseplay, Xbox Live is head and shoulders above PSN. I've never even seen game cheating on Xbox live with my own eyes and I have on PSN... When you have a free account and can get another one easily you just don't care.