There have been a number of members with the same 'issue'. The problem seems to be that, if AT&T allowed you to add further iPads to an existing data plan they are likely to see an increased usage - even if your plan has a limit. This is one of the reasons why they charge additional for tethered accounts.
They have done their research well. If, say, you have a 2 GB account - that allows you to use up to, but not more than, 2 GB per month, their statistics have told them that, on average, a user does not consume all of that allowance. But if they allow additional users - or a tethered account - they have found that, again, on average, users consume a greater proportion of their monthly allowance.
You may rightly feel that, if you've paid for 2 GB you should be allowed to use it anyway you want to. But it's a very competitive business and AT&T have factored the average usage into their costings and can afford to give you a competitive price because they know that, on average, users will not consume all their allowance.
I agree, it doesn't seem fair!! And, for sure, you were not given the correct information on the 'phone..
Tim
Scotland