MarketPress Multisite Users

I have MarketPress installed in a WP Multisite with each 'blog' set-up to require the user to be registered in order to purchase from MarketPress. I then have users register and login from a single front-end login script so that they can browse each 'blog' and purchase items from it.

By default, when I create a new user they are added to the main 'blog' of the multisite and then I have to manual (or by way of plugin) add them to each of the other 'blogs'.

My question is: MarketPress seems to work on each of the multisite blogs even if the user hasn't been added to the blog. Is this the correct behaviour, or does each user need to be added to each blog for MarketPress to work in multisite with registration required on?

Thank you in advance!