IANAL, but as far as I understand it, any given patent holder has exclusive rights to use its invention however it sees fit, including licensing the patent (e.g. patent trolls) or maintaining a monopoly.

Some patent pools (e.g. MPEG LA) are set up so anyone can license the patents covered under either FRAND or RAND requirements, e.g. fair, reasonable, and non-discriminatory, but participation in patent pools is entirely voluntary. In the case of H264 everyone agreed to these terms to boost adoption and the ultimate value of their patents.