Obligatory disclaimer, I'm not an attorney, nor did I stay at a Holiday Inn Express last night. I'm just some guy who's done some research on this in the past. So this isn't legal advice or anything. Consult an attorney if you have a real concern.
The law varies from jurisdiction to jurisdiction. But, in general, you can't be arrested for a misdemeanor offense without a warrant unless the offense is committed in the presence of an officer (or of a citizen making a citizen's arrest), and the arrest occurs at the time of the offense or as soon as possible thereafter. There are some exceptions, but I'm not aware of any jurisdictions where anything related to the hobby would be an exception. Here's how the law works on one jurisdiction:
http://legalupdatesonline.com/4th/172#cont191
If they want to arrest you afterwards, they need to take their probable cause before a judge, and get an arrest warrant. Whether LE would do that in the case of a misdemeanor hobby-related offense I suppose would depend on the circumstances, and how badly they wanted you in particular.
But basically, they can't witness you commit an offense, let you go about your business, then come to your house a couple days later to arrest you for it. At least, not without a warrant.
Now, if LE has probable cause to suspect that someone's committed a felony offense (the only hobby-related things I can think off offhand where this might come into play might be pandering or promoting), they don't need to witness it personally, the arrest doesn't necessarily have to happen at the time of the offense, and they don't necessarily need a warrant to make an arrest.
Of course, if they just want to hassle someone, all bets are off - they could arrest you simply to cause you embarrassment and inconvenience, even knowing that they wouldn't be able to make any charges stick. Or they might show up at your house (in the presence of your family) to "ask about" why you went to a motel and went into the room of a known provider. They know that, even if charges are dismissed (or never even filed), you might still have to spend a night in jail, and be put in the position of having to explain to your family why the police are asking you about being in a provider's motel room, having to explain to your employer why you didn't show up for work that day if they arrest you anyway, etc. This is where the "you can beat the rap, but you can't beat the ride" thing comes into play...