Safer to be a felon than a snitch!
Meanamean:
Absolutely and positively, NO! And contrary to what you have been made to believe by the police officers, not only do you not have to even consider their coercive and downright illegal proposition, you don't even have to take another call from those officers and there is NOTHING, and I mean nothing that they can do to you and you are in the clear as far any future prosecution is concerned. And here is why.
First and foremost, you were not arrested for the pills in your purse and were cut loose after the search and whatnot, and once that happens, you cannot be arrested at some arbitrary future date by the police officers and be charged with a felony for an offense involving the unlawful possession of a controlled substance that occurred in the past. The only way for you to be arrested and prosecuted for possession is if you are caught with them on your person [anew] somewhere down the line and not because the officers changed their mind or the "pills are sitting on his desk" waiting your call-back.
That aside, their threat of prosecution in lieu of you becoming a confidential informant is nothing short of a fishing expedition and in your [particular] scenario and in scenarios similar to yours, it constitutes unequivocally an illegal and a criminal act in and of itself.
Of course, such offers are routinely made by police and the D.A.'s office and are routinely accepted. But those are circumstances in which the defendant has been arrested for a serious parole or probation violation and is facing the prospect of serving the entire and actual prison term which his offense carries and which was suspended by the court pending the successful completion of parole or probation. The defendant is then given a choice to make: either go to prison for a few years or become a confidential informant and remain free.
Here, there really is a good reason for the offer to be made and even a better reason for the defendant to consider the offer and decide on it. But you were not remotely facing anything so drastic as a prison term even if you had been arrested and convicted of a felony possession of a controlled substance which would have been the worst case scenario. But such charges are called "wobblers" which taken together with your lack of a criminal record, would have earned you a reduced charge of a misdemeanor possession and a three-year summary probation.
The officers were out of line to threaten you like they did, but it still remains that you are free to ignore the proposition without any fear of arrest and prosecution for what took place when you were pulled over and don't be timid about telling them to take a hike if they persist and may even table your own threat in kind. And that would be making a formal citizen's complaint and that would mean the automatic entrance of Internal Affairs into the fray to investigate the officers.
Well, needless to say that the decision is yours and only yours to make, but let me leave you with this sobering thought. There are three things to avoid being known for when and if going to jail or prison for or any length of time: rape; child molestation, and for being a snitch!
fredrikklaw