Well, yes, the King has choices too of course. He can create a Letters Patent tomorrow and change the rules over titles etc.
But within a situation that exists - ie reality - you have two parents with two children who at this present time have a variety of titles and non-title social styling available to them.
Their choice completely which ones they want to currently use or not.
Just as it's anyone else's choice to judge them or not for which titles and how they use them.
Personally - I think it's odd they didn't use the lesser titles if they wanted titles for their children, at the time when they were still fully working royals.
When they left to live a different kind of life in the USA it then made total sense to have not used the titles.
Which is why I think this latest move is weird and not necessarily the best option for two American citizens.