How you perceive Pitt's campus strictness is very dependent on what you're caught doing, where you're doing it, and who catches you. In general, campus police and RAs are very strict when it comes to drug and alcohol use. Although RAs are known to be pretty lenient with their floors regarding alcohol, this doesn't mean that you should try and get away with underage drinking in the dorms. Drug use is even more dangerous, and both campus police and University staff are quick to crack down on drug violations. Punishment for getting caught with drugs or alcohol can mean any combination of a fine, a notification sent home to your parents, a required class in substance abuse, and action by the University's Judicial Board. All of these things are completely avoidable if you're mature enough about what you're doing. Most arrests and citations are the result of students being loud and rowdy, or otherwise drawing attention to themselves. Off-campus parties are rarely busted. When they are, those who live in the busted house have a lot of trouble on their hands. In houses around campus, it's probably going to be campus police who break-up the party. There's a good chance that those who don't live at the house will just be told to leave.
City police are the most lenient of all the authorities you'll encounter because they have more important things to worry about than dealing with drunken college kids. They won't often show up to parties unless a formal complaint has been issued. Clearly, the key is to know your limits and not make a spectacle of yourself. Most of the students who find themselves in trouble have done something to ask for it.