I'm not saying Sony deserves this. But based on what they've done, it is natural that hackers would come up with illegal solutions.
By promising Vita is fully compatible with PSP, they
1. force people to buy games again to play on the Vita even if they've already owned those UMDs,
and 2. are not willing to put enough effort to make sure all PSP games are available on PSN, most notably KHBBS and CCFF7.
It is very sad. Because I don't think most people would go piracy if there were a reasonable legit way to play their own games on the new system.
As long as it is possible, such a solutions would come regardless of what was done. It doesnt matter if the Vita had an UMD drive or could run homebrew natively, there is always some reason to hack it, even if it is just for the challenge of doing the hack itself. Same thing goes with games who get pirated, it doesnt matter if the games are DRM free or not, a pirated version is always being put up for download by someone.
Maybe there would be a bit smaller chance of something getting hacked in this case, but it could still happen.
No one is being forced to rebuy the games (i know what you mean however). It is the way things go when a medium isnt supported in the new system. People are forced to buy NES and SNES games on Wii for example, even if they own the orignal.
Why isnt KHBBS and CCFF7 on PSN by the way? They're both 3rd party games, so who control it?