I'm almost finished with Law school, so I know a fair bit about this, but this IS NOT LEGAL ADVISE (I have to say that). The police can go through your trash that is sitting outside, even if it's in your yard. The only protected area outside of the house is known as "curtilage", but there are a number of factors that must be met. The MAIN test is simply whether he had a subjective expectation of privacy, and whether that expectation of privacy is one that society is willing to accept as "reasonable". A trash can outside of your house almost certainly wouldn't pass the "society accepts as reasonable" test.
If they found syringes and empty gear bottles, that would almost certainly be enough to get a valid warrant. All that is required for a warrant is "probable cause" (a substantial likelihood), and a warrant issued by a neutral and detached magistrate, which it sounds like they had. Since they were tipped by your roomate who sold weed, and found paraphernalia, the scope of the search allowed would be almost unfettered. The warrant MUST state with particularity what they are looking for, and the scope of a constitutional search must be strictly confined to places that those items could possible be (ex. if it's for a stolen piano you can't open any dressers or drawers, etc). Since they were looking for weed or gear, well that could be just about anywhere.
They may not care about your small amount of gear, and they'll surely tell you that, but take eveyone's advise and DO NOT speak to them ABOUT ANYTHING without an attorney present. First thing out of your mouth needs to be an unambiguous, "I don't wish to speak about ANYTHING w/out my attorney present". Then they MUST "scrupulously honor" your request, and anything you say would inadmissible under the exclusionary rule anyway.
I hate to say it, but they were justified to do what they did, at least under the law. But a good lawyer should be able to get rid of this. Just don't get the name of a guy off of TV, or the biggest ad in the yellow pages, ask around and get a good lawyer. Since they didn't find much worth pursuing they may just let you go with a warning, or maybe a small fine. If you're in a big city of a small town might matter too. Good luck. You can PM me if you have any specific questions, I'll answer them of I can. I don't check the board that often though, I'm actually taking finals right now.