In USA, is it legal to hear police or ATC (air traffic control) frequency?
In the US the answer is generally yes, you can listen to such transmissions, but local and federal laws may have some restrictions on what you can do with information gained from listening activities, and in some very specific circumstances you aren't allowed to listen at all.
Most of the time the restrictions only come into play when you divulge information you gained from listening to certain transmissions.
But there are also restrictions that disallow intercepting transmissions for one's personal gain, such as operating a taxi service, listening to the competitor's dispatcher, and acting on that information to gain a business advantage. Alternately, there are restrictions on trying to obtain for-pay services without paying for them in certain circumstances, such as pay-tv, hold music services, and so forth even if they do not encrypt their transmissions.
While it's generally believed that listening to cellphone conversations is illegal, that is not generally true, and is not federally mandated. However, if you sell scanners you must restrict them from receiving in the cellular bands, for instance. Also there are specific harsh penalties for divulging information you may learn from cellular transmissions. This makes it difficult to listen to them, or profit from listening, but doesn't necessarily make it directly illegal to listen to them. There are additional restrictions on decoding digital transmissions, which may make it illegal with today's systems.
Local laws often make it illegal to listen to law enforcement transmissions in connection with a crime.
However, Supreme Court rulings, FCC rules, federal laws, and local laws are open to some interpretation. The legality of listening to a particular transmission will depend on more information than you've provided.
If you are a licensed amateur radio operator, using your Part 97 gear, the answer is an unequivocal "yes".