Reasons I can think:
1. Its a VERY small market and nobody cares to push it, if you want to be an a$$ complain.
2. It is hard to find real 3.1 ROMs! Especially today, you might find some on Vesalia or AmigaKit, but stock is usually low.
3. Who owns what? Nobody seems to know anyways.
4. But to get completely technical, yes it is pirating. But it is 2013. Lets just be real here. Nobody cares for 25+ year old computer equipment.
I can see it now, can you imagine people bitching and complaining over 1986 PC BIOS copying/selling on vintage 8088 and 286 PC forums.... lol NOT
Only Amigans make that possible!
EDIT: YEP AmigaKit does not sell A600/500/2000 Kickstart 3.1 ROMs
http://amigakit.leamancomputing.com/catalog/index.php?cPath=40_99