I may be wrong about this, but I believe union lawyers are typically used for disputes between an employee and the company. This is likely to be a criminal case, and I’m not sure how the union’s legal representation would work in that situation.
Police union lawyers defend cops, I’ll bet there are many lawyers on retainer to defend union members in cases like this, we’ll see.
UNions have liability clauses up to a point their lawyer will cover you. When you opt out of joining you also opt out of the liability insurance and attorney