Do all police officers belong to a union?

Depends on the jurisdiction. Some US cities or states require their officers to be in a union, some do not. This applies to many jobs, as it happens- some states have it in their law that you can't be forced to join a union, while others do not.

As to why one might choose not to be in one: often it's a bad idea to not join the union, but officers who are very politically opposed to the idea of unions may refuse. A few may also feel they don't need the extra protection.

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No. Union membership is voluntary. Also, unions are for the lower ranking officers. Once you are part of the management team, you are no longer in a union.

In some right to work states, they were allowed to charge those who choose not to be in a union an "administrative fee" to cover the union's time during collective bargaining. Here is a link to that law:

http://www.nrtw.org/a/a_1_p.htm

My state recently changed the law, so if you don't join a union they can't charge you the administrative fee. Regardless, we have 100% participation because the union will protect you if you are sued. If you are not a union member, you may wind up spending thousands on legal fees.

It depends in many cases, YES in the larger towns and cites , unless the given state statutory law prohibit it.

In Louisville,KY., the local cops have to belong the FOP, Fraternal Order of Police . LMDC( Louisville metro Department of corrections) sworn officers have a Corrections FOP lodge.

At LMDC the non-sworn Sargent -level Supervisors belong to a Sargent Union.

In both cases they have to belong to the union in order to work but not in the case of the non-sworn LMDC Sargent level supervisors.

In KY ., the statuary laws prohibit an union for the sheriff deputies and the Sheriff 's (civilian ) non-sworn staff are considered "at will" employees of the sheriff and can be fire as such.

Some do and some don't it varies by department. Example New York officers are in a Union. https://www.nycpba.org/