My question is, can the police charge you for refusing to help them in their cases if you in fact have done nothing illegal to start with?

I was at a Publix today, where someone approached me asking if I wanted to buy his foodstamps. I responded no, and continued about my business. A few minutes later I was approached by the manager and someone in plain clothes who identified himself as a police officer and asked me what was that guy asking me. I told them what had transpired, where I was told next that the guy was standing outside the store, and that I needed to approach him and tell him I wanted to buy his foodstamps. I told the officer, no I didn't want to get involved, where he responded I didn't have a choice. I said again I did not want to get involved where he appeared to get angry and said I already was involved, and that at that point I could be charged with a criminal conspiracy since I admitted I had discussed buyingthis guy's foodstamps with him. I responded I did not discuss it, he asked me if I wanted to, and I told him no. Then he (law enforcement officer) with managerapproached me and asked what was discussed and I told them the truth of what transpired. How am I involved in a criminal conspiracy? The officer give me a hard look and asked if I had ID on me. I said yes, and he said to give it to him, which I complied. I was then taken to the office, patted down, and then handcuffed while he ran a warrant check on me. When it came back with no warrents, he took the cuffs off, but told me if he wanted to, he could charge me obstructing a police investigation, but since I had no record, he was releasing me, where he wouldn't have if I did. I was then released, where he informed me that he was going to remember me and that I refused to co-operate, and will keep it in mind the next time he encounters me, and that I used my get out of jail free card with him. My question is, can the police charge you for refusing to help them in their cases if you in fact have done nothing illegal to start with?