IDA gives away x86 and charges for x86-64, maybe they're going for a similar freemium model?

I think the problem is technical rather than commercial : https://github.com/avast-tl/retdec/issues/9.

x86_64 have calling conventions (namingly __fastcall) which are more inconvenient to decode than x86 _cdecl or __stdcall where every arguments are passed on the stack. Most symbolic engines usually works only on x86 for the same reason.