Scirra store is part of old website - plan is to move it over to Construct.net eventually.
We could of sold the license through Scirra.com, but it would be very difficult and time consuming to add. Looking long term, everything is moving here so decided to go this route and split it off for now.
RE credits earned for selling items in the Scirra store, we do not and probably will not ever allow you to spend this credit on your subscriptions or other items in the store. The reason for this is although it seems like a good idea - from an accounting point of view this is very hard and time consuming to process especially when sales get refunded, the original sale is charged back etc etc. Basically introduces a whole host of extra edge cases which are very hard to account for, and time consuming to accommodate!
Ultimately it would cater for a minority of users, with a disproportionate overhead from us.