These parents are serious about curfew. You'd think you'd just give the kid a key if it was that much of a big deal for them to come home late, but a parent's ways are mysterious. By not giving their kid a key, they hold complete control. At the end of the day it's all about power. You have to show your kid that you have the power. "You may leave the castle... but you may never come back!"
As someone who has come home smelling like weed when I was a teen, this method is actually preferred. I'd rather not have to deal with stinking the whole house up by coming in after a late-night hot box. Plus if I'm already sleeping outside I can just toke up a bowl in the middle of the night.