Why is tethering not allowed?

The plan is NOT throttled at 22 gigs.  After you hit 22 gigs the line is reprioritised. Many users notice a slowdown after 22 gigs, some only occasionally and some never notice a slow down.

In order to manage the network for all users, limiting use to one device only is the choice that works for ATT.  Verizon is now in the process of terminating accounts due to over use and abuse of the unlimited plan.  The other carriers have drawn the line where it works for them.

 BTW, you might want to read all the fine print on T-mo plan.  

Tethering is at 2 G speeds only.

I doubt anyone knows why other than it has always been that way. The old unlimited plan didn't allow tethering either. By the way you do not get automatically throttled at 22GB. Your data traffic is re-prioritized which may or may not result in a slowdown. Many customers have reported using much more than 22GB without any reduction in speeds. Perhaps that is the main reason for not allowing tethering. 

You can all tether. You just need a plan that allows tethering.

That's the way they sell it here.

You only want 9 eggs, that's fine but they sell them in a dozen. That's the choice. Does it matter why it's 12?

AT&T does NOT want to sell you unlimited tethered data. I'm sure there are countless reasons and ways they are worried about abusers.

They could set a limit for the amount of tethered data and then everyone would argue what the number should be.  Or make it an additional charge and then hear complaints about that. There is no winning for them...

You ask "Sure I have great coverage with AT&T but what's the point since I can't utilize it."

I'd ask the other side of the coin; what's the point of having all the data you want (unlimited) with TMo if you can't get a signal to use it? You admit their coverage stinks.

Besides, as two posters have said, you may never experience a slow down of your data.