Because a person can be responsible for more than 1 domain with different settings. For example, if he/she is a member of different social communities (that are not connected in any way) and has decided to have these sites hosted with me as a reseller
I think it is not customer friendly (or how do i put this in english) to have a different user account for each domain.
like you said "as a reseller".
Create a reseller account for the user, and he can choose his own packages.
put in a maximum domain limit and he can't create extra domains without contacting you.
Why not just let the customer have multiple add-on domains, that way they can host more than 1 domain and web site on their account, and it would be the same as you giving them multiple packages.