free market forces is great when the is information symmetry between buyer and seller. In that world no person would ever be surprised when the phone bill is sent.
Now the question is, how many consumers knew roaming prices (for places which they would travel to) when they had the opportunity to compare companies and made the decision to enter into contract with a phone company. If the answer is almost everyone then regulation is completely unnecessary. If the answer is almost no one then competition is basically impossible and the consumer just have to hope that the contract they signed several years ago was a good one and not a exploitive one. Regulations in that case reduces risk to the consumer.
There was another issue: most people don't travel very often, so roaming prices weren't even considered when signing contract with mobile operator. And when you do finally decide to go for that dreamed of vacation abroad you won't be switching your operator just for that purpose. So you just agreed to whatever roaming plan your current vendor had to offer. And then used your phone only for absolutely urgent things while abroad. Or bought a prepaid sim card from any of local vendors.
Put another way: most consumers don't care about roaming; if they did they would consider it when signing a contract. So why should the EU force roaming on everybody instead of letting operators offer products tailored to the individuals?
Whether the market is currently competitive or not, this move will result in less competition as there is more price fixing and less room for operators to differentiate their offerings.
Now the question is, how many consumers knew roaming prices (for places which they would travel to) when they had the opportunity to compare companies and made the decision to enter into contract with a phone company. If the answer is almost everyone then regulation is completely unnecessary. If the answer is almost no one then competition is basically impossible and the consumer just have to hope that the contract they signed several years ago was a good one and not a exploitive one. Regulations in that case reduces risk to the consumer.