Junta oddly defensive of charter

It seems very strange to me that General Prayut Chan-o-cha is telling the Future Forward Party it cannot promise to change the Constitution if elected.
As I understand it, the charter has changed many times. The current one was written by the non-elected military government, which never signalled its intentions to make such a sweeping reform, nor to set laws that will inhibit the actions of elected governments well into the future.
In a democracy, parties make promises and, if they are elected, they (hopefully) enact them. Making a campaign promise should not be illegal. And at least Fast Forward is being transparent about what it wants to do in government, unlike some other new parties which are clearly there to act as proxies for Prayut.
The general, whose own political ambitions are obvious now, seems not to understand how a true democracy works. Or, perhaps more accurately, he does not want to understand it.
B Davis