Well, it's for the same reason people rob graves. There is money to be made off of of the corpses of nostalgists. Frankly I think paying hard cash for ROMs I already own to be hardcore shhmitty. I would call the people who own the software rights to the kickstarts petty, but that would be like calling a murderer a "willing life taker with poor motives".
"FIND" the ROMs that you own (if you have an old A500, download the kickstart ROMs and leave your guilt at the door) and play happy. Usenet is a brill place to start. Or simply Google them... kick.rom amiga
Go get them, tiger.