Unfortunately not everyone responds to Kuvan, so giving this medication to every patient with PKU is not possible. If I remember correctly Kuvan only works for approximately 30-50% of individuals with PKU.
As for people who use Kuvan being able to eat whatever they like, you are right that this statement is incorrect. Kuvan is suppose to be used with a low protein diet, not in place of it. However, some people are able to increase how much phe they can eat if their levels become too low as a result of Kuvan. This is entirely at the discretion of the patients doctor though.
Other advantages to Kuvan that I have noticed since beginning Kuvan nearly four years ago include more energy, the ability to focus more, better temper control, and more stable phe levels.