User site to have subscription

May be I'm missing something here.In my WPMS 3.1 when a user tries to register from a user created blog,s/he is redirected to mainsite.tld and no way to add him/her as subscriber to the child blog alone.
I have a found a similar answer here:
http://wpmsquestion.com/advanced-questions/relative-wordpress-multisite-user-registation/
Unless we are creating a network then what is the point of this I wonder!!It might be confusing for end user and also the site owners would not love to share it's users among all sites in network right?
Will Membership plugin be able resolve this issue,I'm unable to find any option to sort this out!?!?
Kindly help.