mod 'ownership' and copyright protection

I'm curious:

If a company sells a software product, and as part of that software they permit (heck, even design it to be) modification of certain files, in order for the gaming community to 'tweak', expand and improve:

1) Isn't it implied that such modification of their files is for public free usage? That the files still remain proprietary, and making money from their software is illegal?

2) If I recall copyright law correctly, if person A modifies software (given that the owner of the software permits such) and presents it in a finished, tangible form, than that tangible finished form is copyright protected against person B copying it and selling it under their name. So why can an online company harvest and steal our modders works, claim it as theirs, sell it, and not be in violation of copyright laws?