why is it like whenever i join a PC running windows 7 using the user's AD account, is prohibited instead of using our Domain's username and password?
I do not understand what is asked. What is exactly the difference between "user's AD account" vs. "Domain's username and password"?
This is the basic principle of domain administration that machine can be joined to domain by domain administrators nut not by AD "common" user accounts
Are you using a domain user account and not being succesful, or are you trying to use a PC's local account to join it to the domain?
In the first case, there could be some policy in place which only lets some users (probably domain admins) join machines to the domain.
In the second case, you're doing it wrong (TM): in order to join a domain, you need to authenticate against it, so you'll need to use a domain account (and possibly an administrative one).