Because those games don't use WoW accounts, duh.
The only acc they share is your b.net acc. You can have multiple WoW accounts on the same b.net acc and log in on each of them w/o any issues, that's how multiboxing works. You, however, can't log in on the same WoW acc multiple times, if you try, you get kicked out.
Classic and retail WoW use the same WoW account for logging in. You pay one sub fee for one account for both versions of WoW.