Certainly if they have no evidence on you, than of course it was illegal, but there need not be anything more than probable cause. Now, you don't mention if they searched you and found something that was not paid for. Your argument certainly would be stronger if they did not find anything. They are supposed to ask you if they can search you before they arrest you. I think that would be the only argument you could address. As you know, people who just drive on the freeway in Forida can be stopped on the mere belief that their vehicle looks like a it could carry drugs. There seems to be a lot of loop holes on ciitizens rights at this time, especially since the new Patriot Act came out, they are using it for all types of excuses to invade citizens. It's scary.