It's not very different, both are under law, just one is under contract law.
Of course you can own things that you can't sell anyway, you can own an extended warranty on your flatscreen television, or own non-transferrable bonds in a company.
Most things you own you can sell, but it's a mistake to think that ability to sell is a prereqesite to ownership, and a non-transferable licence is one of those things on the list of 'things you can own but cannot sell'.
And on the 'buy' point - Do you think you can buy a haircut, or buy a pedicure?
If we are happy that you can buy services, I assume we are happy with Steam using “buy” to represent the purchase of the service and licensing offering they have?
