Well, the rule when writing APIs is that the public interface should be as small as possible! It's easy to change your mind and make something public, but going the other way is harder..
In the case of the bool/float/etc classes, they're designed to be simple, and I hid as much as possible to avoid beginners getting confused over how to use them. If you need to tweak the raw parameter values as well as their normalised values, then maybe you have a use-case that goes beyond their limits, and you should be writing your own class instead? There's very little code in there, and I expect many people will want to use their own classes.
There is an argument for making the getText method public though.
But with the ValueTree one, it's all deliberately hidden away - the point of that class is that your program interacts with the ValueTree, not the parameter objects themselves.