I never saw why this interference in the free market was needed when it was making progress, and in some circumstances already provided solutions to the problem on Three I've had free roaming for years, at no additional cost, across the EU and a good chunk of the rest of the world http://www.three.co.uk/feel-at-home
Your point about Three is actually a reason why this agreement was needed. Why have none of the other carriers done the same? They have had years to act on this or make some effort to reduce the costs of roaming, yet remained complacent.
Don't assume the actions of one carrier represents all of them. Three is a great carrier but their market share is small compared to others. Other networks likely didn't make any efforts to reduce the costs for customers because they didn't need to. Additionally, Three's motive may be more marketing and attracting new users than simply for the good of their customers.