Was stopped by a police officer and given a written warning for not having plates on my Motorcycle.?

I recently purchased a brand new motorcycle for myself. I live in Illinois and purchased the bike from a dealer in Iowa. They had agreed to do title and licensing, however could not do so because of them being and Iowa and me being in Illinois. Upon delivery of the bike my dealer gave me the purchase agreement saying it was good for 45 days to ride on without registry or plates. Recently i was stopped by a cop who said he noticed i didnt have my plates (He obviously had it out for me since the second he passed me he pulled a uturn) I told him what i was told by the dealer, handed him my purchase agreement and license and then waited for ten or more minutes while he was doing stuff in his car. He came back and told me he was giving me a written warning for not having plates. I told him what the dealer told me. He casually brushed it off, and i didnt push it becuase i didnt want worse than a warning. Is this legit? It seemed like the cop stopped me becuase he had nothing better to do and wanted to get me for someting (I have no doubt he was checking my background looking for warrants etc. in his car which is why he was taking so long.) So does anyone know their stuff on this subject? I need the bike to get to work, and would like to ride it since it is in my possession. So, am i riding legally with my purchase agreement, Drivers license with motorcycle endorsment, and insurance? Or did my dealer not know what he was talking about?