Design Philosophy
Licensing

software which OpenBSD uses and redistributes must be free to all (be they people or companies), for any purpose they wish to use it, including modification, use, peeing on, or even integration into baby mulching machines or atomic bombs to be dropped on Australia.

Kernel code must have a BSD or similar license
Non-free clarification of ipf license triggered it's removal and PF develeopment
Preference for 2-clause BSD or ISC-tyle license

PF is licensed under a 2-clause BSD license.
Allows PF to be used in commercial products