First and foremost, thank you for choosing justanswer.com. I am a licensed criminal defense attorney and member of the American Bar Association and I believe I can help.
Your intuition serves you well when you believe this is wrong and should not be happening. A person loses their rights to carry a driver's license when they lose their license; nothing more, nothing less. The police aren't able to come and remove your license plates simply because you no longer have a valid driver's license. This would be akin to them doing such for an individual who simply allows their license to lapse, but keeps their car in their garage.
Unfortunately, it does not sound as though the police in your town will give you the plates back just by you saying "please." Here is what you need to do; and you don't even need an attorney although having one would be easier:
File a motion for return of property with your local court clerk or prothonotary. This can be done at your local courthouse. State the facts in the motion (i.e., you lost your license and the police came and took your plates). The State will then have an opportunity to tell the judge why they feel taking your plates was a legally viable option. The judge will then order the police to (a) give you back the plates or (b) that they have acted according to law.
As stated before, it is my opinion the police have absolutely no legal rights to your plates and, thus, you should be able to win this in court with little argument.
I hope this helps! Thank you in advance. Please, click "Accept" if you found this answer helpful so I may be credited for my response. Good luck to you!!!