> Why not hit them with theft of service for continuing to draw power and use public infrastructure?

Not an alpr apologist but aren't they almost always solar with lithium storage?

even if they were totally off-grid, are there any other municipal functions you can use rent-free to put up private infrastructure of your own without pay?

tl;dr : there is a cost associated to allowing a random company to cement down a galvanized steel pole somewhere in town; for installation, removal, and liability.