The current behaviour is probably not what a new user expects.
I can't think of any other program that behaves like this.
Shell prompt for example do not.
I agree with Drew that the default should be that "Inviolable"
should be on (it should not be possible to move the point into the
minibuffer prompt). Is not that in line with how we have thought in
other similar cases?
It is useful to move point into the minibuffer prompt without
changing default settings. But since this can be confusing for
beginners, maybe it should be more difficult to move point into the
prompt area by default? For example, to disable moving point into
the prompt with C-b, but allow C-a at the beginning of the input
area to move point to the beginning of the prompt. This is like C-a
(`message-beginning-of-line') works on message header fields.