If the sale was made via visibility and one click transaction on THEIR App Store 100% they deserve a sales commission.The issue is, if you sign up to say Netflix on your iPhone, do you think Apple deserves 30% of that? Or do you think you should have to pay 30% more just because you're an iPhone user than other platforms?
If I sign up via a browser from an advert I've seen elsewhere, then no.
Advertising is not free and arguably the single most expensive part of a product.