The police were correct in claiming that all citizens are unconditionally obligated to obey a police order "in the name of the law," even if they suspect the order is partially or entirely unlawful. Of course, citizens have the right to file a complaint later.
Logically, this is a hard precondition for a police to actually being able to do their work. Imagine, that any person feel the right to question the police request.
I applaud the Czech police to be so gentle with this guy.
