Withholding tethering from the 2gb plan helps them reinforce their claim that tethering is a feature separate from regular data. If they allowed tethering on the 2gb plan, it would strengthen the claim that all data is equal. It would make it more difficult to justify restricting tethering from unlimited plans.
Not really. I see no difference between 2 GB vs 4 GB. There is a difference with any cap vs. unlimited. What restricts unlimited is the screen size. Dealing with a phone-size screen gets old/annoying after a while. But tethering with a laptop makes everything easier, so one will naturally use more data.
With a cap (2 GB or 4 GB), that doesn't matter. Because of the plan's cap you'll still stop at those limits regardless if it's hit via the phone directly or a tethered laptop and if you do go over, you'll pay for it. The tethering fee is simply "free" money to ATT.
Anyway, I can't justify paying $80 per use for tethering each year ($20 x 12 months / estimated 3 times I'd actually use it).