You are not paying the roaming charges if you unlock your phone. Thus breaking your agreement.
Credit card companies lobby to make debit cards illegal
Yes they want you to overspend. Their revenue strategy should not depend on it.
This is a very common contractual scenario--you get a discount for agreeing to only use one particular vendor, to the extent that you have a need for the particular kind of service they provide.
As per your binding contract, you agree to only use their service in return for getting the phone at a subsidized price. Their incentive to charge you less for the handset (incurring a loss on the sale) is that you agree to guarantee doing business with them.
And if that's a problem, unlocked phones are still always an option.
You mistakenly believe that contracts are in perpetuity.
In which case I'm sure we'd all consider it reasonable to first ensure AT&T covers it's cost by increasing its contract buyout amount. We're talking here about the price of roaming being artificially high. You step across the border and come back to an inflated bill because of termination 'agreements' between providers.