> free software wants to strengthen copyright. I would have thought that most were against IP in general

That's a misunderstanding.

In a world without any concept of authorship and copyright, people would share, modify and redistribute books, drawing, musical scores...

But most software can be compiled (and even obfuscated), making it unpractical to further modify and redistribute a copy.

This means than in such word closed source would be still present. And freeload from FOSS without even giving attribution.

FOSS is about building a cooperative development model.

Copyleft is based on copyright laws simply because there's no other legal instrument to ensure that software remains open.