Why do registrars force you to use their nameservers first?
It seems like when I register a domain I have to go in and change the name-servers, then wait 48 hours for that to switch over, but why do they do this? Why can't they let me choose the name-servers prior to registering and start off with the right settings?
For 'consumer' registrars, it's probably because 95% of their customers are small non-tech businesses who just want a website and don't know what DNS is, etc.
Nowadays, name server changes for well-maintained TLDs usually propagate within an hour or so based on my experience.
They don't. Good ones let you choose at registration time. Mine lets you pick a default in the first place of your own..
Perhaps they have been "parking" the domain previously, very common amongst some large registrars