The language here needs some clarification: the GPL (or other free copyleft license) only "forces" someone to do something under very particular circumstances, which could be characterised as:
1. They are planning on distributing non-free software, and,
2. They haven't made a special agreement with the copyright holder.

I would hope that condition 1 does not hold for a large proportion of library users.
Those users should not be scared away from copyleft.
Using it has the advantage of offering a reason to push those on the fence about whether to make their software free.