Checkpoints are legal in all the states I've lived in as long as they stop and at least do a cursory check on every vehicle.

My last "offense" (yes, I did something not illegal in my home state, but illegal where I live now) went much the same. Discussion turned form my offense to the range bags in the back, and getting stuff out for show and tell. I got a verbal warning, but the officer stops at the same coffee shop on Saturday, so he'll remember me if I trangress again.