The US and all state's constitutions protect from unreasonable searches and seizures. If the search and seizure is indeed unreasonable because of lack of probable cause, that taints the evidence and can lead to its suppression...NOT to a dismissal of the charges per se. But even if the evidence is tainted, that taint can be purged if the person was in fact committing some other crime even if it has nothing to do with the reason you were stopped or the reason that evidence was obtained. This means that if you've got any expired tag or a defective tail light or anything that would constitute a traffic offense, in most cases it will purge any taint to the stop and subsequent investigation. So if your tag was expired, their stop is fine even if they subjectively just stopped you to profile you, or for no reason at all.
Now if they did stop you for an expired tag, then that in and of itself only gives them the right to investigate circumstances related to that traffic offense. So that's not sufficient to just go searching everybody. They'd need probable cause for that specific search. If the officers smelled marijuana they could search for drugs, if they could articulate that you might be carrying a weapon, they could search for weapons, etc... The fact that you can from an area that they knew was involved in drug activity could give them that justification too.
But the bottom line is that you are not being charged with possession of anything. There was no evidence that was seized. If that were the case then maybe there would be a suppression issue, but there's not. Freedom from unreasonable search and seizures just gives you the remedy of suppressing evidence unlawfully obtained.