If your vehicle is out of compliance with law - and a front liscense plate is required in many states - the police can detain you for the violation. This is an "arrest" and during this arrest, you must comply with the instructions of the police officer.
Period.
This is the wrong time to voice complaints, and certainly the wrong time to start a confrontation with 2 guys with guns.
Police work is a difficult and dangerous job, made more difficult by [censored] that act like that in these situations.
I'm not an apologist for police misaction - far from it - but when they are detaining you is the wrong time to be a combative about it.
You might find this interesting:
http://www.YouAndThePolice.com
In short - never give up your rights, never admit to anything, but at the same time, don't give the police a reason to taser you.