That's a tremendous amount of power for one organization to have. Imagine that the majority of online purchases eventually goes in this direction and then Amazon decided that it's going to charge the sites it represents 5% for each transaction. The sites cannot refuse because Amazon has become indispensable. At what point will Amazon be so big that they violate anti-trust laws? Obviously this is presumptuous, but not unlikely.
It's different for a number of reasons, not the least of which is the global nature of these internet behemoths. Increasingly these large organizations are seemingly stateless. Consider Peter Theil's (Paypal) proposed Libertarian Island (you can read mk's take on that here and it seems to me that they will eventually exist outside of regulation. That's a scary thing imo. While we all want a free and largely unregulated Internet, we also don't want a select few handling all the data/money. Bad stuff could come of such things.
Hmm...I would suggest that Esso and AT&T were (are?) essentially stateless in the same way. Any corporation of sufficient size has the capacity to overcome government regulation at some point surely? The difference now to then maybe that today's entities have a much more effective global reach due to the massive enhancement of communications, but I don't think we should ignore the fact that the big oil companies used to (still?) routinely overthrow governments which were in the way of (their) progress?