Can anyone buy a R1200RT-P or do you have to be in law enforcement?
A US R1200RT-P, is a US R1200RT variant, electrically it has a R1200RT-P wiring harness, two batteries, larger alternator, an additional US wiring harness, and a few other things. Cosmetically it has protection bars, "Authority" side cases, additional light brackets and a 1-up seat/radio box configuration. As far as the power train goes, it has different valve specs, gearing & a engine oil cooler. It also has slightly different software for the various controllers (ZFE, Engine, etc.). Standard US paint colors are Black/White, Titan Silver & Barritz Blue.
Generally anyone can purchase the motorcycle described above, except (think) in California a new black & white vehicle may not be sold to the masses.
Some long distance riders are attracted to the larger alternator & dual batteries.
Other items like lighting is dependent (in the US) on the laws and regulations where the vehicle is registered. There are companies/people who own RTs/RTPs who are not law enforcement that perform escort duties for funerals, bicycle races, large equipment, among other things (a few EMS providers) and their lighting, etc. conforms to local laws.
Putting lettering that says "POLICE", "TROOPER", etc. would generally be considered in extremely bad taste and probably ruin a nice ride in the afternoon.
What ultimately makes a fully equipped police motorcycle is the sworn motor officer. The RT-P just happens to be a variant on the RT, with features attractive to law enforcement.
If I missed something please chime in.