Of course they can sell transit. The reason they don't is that it has
the potential to create a conflict of interest. When your customer is
also a competitor and your customer suffers an outage that's your
fault... Well, you see where this is going.
Over the coming years, I expect exchange points to do some strangely
interesting things, as the towel of revenue continues to get tighter
and tighter to squeeze.
Especially so if a few of the large content providers continue to
pull back from route servers and such.