Marijuana use alone doesn't cause any problems for anyone else. Period. Your question assumes that it does and that's just plain wrong. However, marijuana use followed by certain actions can cause problems for others. So those actions while intoxicated are what should be illegal, not marijuana use by itself.
It's like you're asking why should we legalize knives when they can be used to stab innocent people. Well knives aren't the problem, irresponsible use of knives is the problem. Just like marijuana isn't the problem, irresponsible use of marijuana is the problem. So the irresponsible use of marijuana is what should be illegal, not the entire substance altogether.