Gamanit: While what you said is true, I assume that access in a locked room is much much much much more dangerous than using the library or even an internet cafe.
As I said, parents should be the ones providing the wi-fi, and seeing what their child is looking at. Parents should use common sense though- if a kid is spending hours in a locked bedroom, the lock should be taken away. If he/she is simply locking the door for a half hour in the morning to get dressed, and the same at night, I see no reason he/she shouldn’t have one. As much as you may knock before opening your childs door, it’s still uncomfortable getting dressed when the door is not locked.