Thats a good point you raise. But at the end, there is the ultimate reason that Apple has: "Just because other Apps are doing this, does not mean you can do this too." I think they have put "cloud storage" into a category, that has to offer IAP. Since they made Dropbox and Google Drive offer IAPs already, they expect every new entrant to follow in line. I am sure they will start targeting other SaaS markets as well.