It is reasonable for an officer to arrest someone for a misdemeanor for several reasons.
The most important reason is because someone has broken the law. This is reason enough. If people select what laws they want to follow and not follow, or when they want to follow them, we would have a chaotic society. Law are given, so that people will follow them. People need to understand this.
Second, often times when a person gets in trouble with the law, then it can act as a deterrent for future crimes. Also others those who see that the law is upheld may think twice as well before they break the law. I actually believe that society should enforce the law more consistently.