(Disclaimer: IANAL, this does not constitute legal advice, seek a lawyer for more guidance if necessary)
GPL and other FOSS licenses are only concerned with distribution; indeed, restrictions on use are forbidden by free software and open source definitions, it can only enforce restrictions on distribution. So, GPL clauses come into force whenever a GPL program is distributed to other people.
The thing that's at issue here is the FSF's interpretation of copyright law when it comes to software and derivative works.
- First, the FSF takes the position that a program that dynamic links against a library at runtime with constitutes a derivative work; you've combined a program and a library to make a new program.
- Second, "collectivity": They take the position that when you distribute said program with said library, you are distributing a "combined" program that constitutes a derivative work, even if said combination actually happens at runtime. The argument is that when you distribute them together,