If the games are using the music in violation of copyright and valve acquired it and is selling it, the distribution rights part of copyright law allows the copyright owner to sue anyone in the supply chain. If the music was originally licensed properly, then PRS is just looking for a settlement.

Once you purchase a legally distributed work, you are protected by the first sale doctrine, which allows you to do pretty much whatever you want with your single copy.

Is a Paregrine falcon any relation to a Peregrine falcon? They look remarkably similar.