Sony are guarding against a scenario where people are using their console, platform and network infrastructure (account system, friends system, party system etc) to play a game but then, when people go to pay for things inside the game, they get no revenue because the user buys elsewhere. It's not difficult to understand that this undermines the business model for those platform holders, which is why Apple and Google are in legal battles over it, and it's why Sony have put in place these clauses.
If I go into a shoe store, take a pair of shoes, and then pay for them in a different shoe store... that would be weird right? That's at least as close an analogy.