You'd have to ask a lawyer if this would fall under probably cause - i.e. a police officer is allowed to arrest someone if they have a "reasonable" suspicion that a crime was committed, and then the prosecutors decide whether to bring charges. It's not clear to me how a court would view an officer being mistaken about the law in specific ways.
This depends upon where you are doesn't it? In the UK the grounds for arrest are "reasonable suspicion" in the US isn't it "probable cause"? I think that "reasonable suspicion" is similar to 10% certainty and "probable cause" 25% (c/f with "beyond reasonable doubt" which should be 95% certainty)...
I am not a lawyer. If anyone is in doubt I suggest speaking to a lawyer!
"Probable cause" is defined using the word "reasonable" in lots of sources. The root of any UK/US difference is more likely related to drift in precedent over the last couple of centuries than to any precise difference in the meaning of words.
