Rather than being helpful, his behavior wasn't normal.
You're probably correct. If eleven horn-honking, perturbed people showed up outside my house at 3 a.m. & demanded to speak to my 17 YO son, they'd have been greeted with Ole' Bess.
These people don't go to bed like normal folks, remember? JVDS wasn't even home at 3:00 a.m., which was another school night. And since JVDS was the last person she was known to be with, I'd think any decent human being would be glad to cooperate and help Natalee's desperate and frantic parents unless they had something to hide, but that's just me.