increased usage means more cost for the provider. How does that offset the income loss?
Let's say the carrier currently charges EUR 1/MB for a service that costs them EUR 0.02/MB to provide, and customers use 1 million megabytes. That's EUR 20,000 in costs and EUR 980,000 in profit.
Then they are forced to charge their domestic rate of EUR 0.10/MB for roaming data, and customers stop being stingy and use 20 million megabytes. That's EUR 400,000 in costs and EUR 1,600,000 in profit.
Obviously these numbers are plucked straight from my ass but surely you can see how it's possible. Roaming charges are almost pure profit as it is, and that's only possible because we're a captive market.
P.S. What is up with Slashdot still not being able to display the Euro symbol (â)? This is 2014, isn't it?