It's mentioned that tampering with past events is illegal in the Wizarding world. Hermione even states "We’re breaking one of the most important wizarding laws!" when they are using it.
The thing is, if tampering with the past is illegal, how does the ministry enforce it? For example, if a wizard were to travel back in time, effect a change, and then return to his origin place, with everything around him changing, wouldn't he be the only one who would have been aware of the change in the first place?
Obviously the Ministry seems oblivious to its use, as it was used illegally to make a change to the past and free an extremely "dangerous" criminal (Sirius), also a big no no, with no repercussions to its users.
Furthermore, if an official from the Ministry were to detect illegal use of a Time Turner somehow, could the wizard or witch who was caught not simply use the Time Turner once again, travel back in time, and do something in order to stop the official from noticing the use of a Time Turner? How would they even punish the user for breaking the law if they were to detect it, if the user could just time turn his/her way out of being apprehended.