If roaming is free, can't you just use another countries cellphone provider? And if so, it's likely the cheapest country needed to raise rates should people from more expensive areas do so.
So, they still can charge lots to those who, getting warned, don't immediately stop using their phone.
It is complicated because this is a political compromise. Everybody knows where the EU wants to end up (the frequent use of 'may' in that answer is a nice indicator), but that means shifting income of operators around (in general, operators in popular holiday countries lose a substantial source of income), so it can't happen in a Big Bang operation.
Anyway, for a typical one week vacation this is a good step forward.
You're paying for 300 minutes at home? When you're abroad and phoning to a EU number, you're using those 300 minutes. Before, you either had to get a special package, or pay roaming charges.