Loosing? Learn the English language, people!

This is the crux of the matter. You're barking up the wrong tree with parliamentary language and imperatives. "Why doesn't she be honest with the British people..." implies to me (in the context) that she'd have to take a specific future action (saying how she plans to retain it). "Why isn't she honest with the British people..." implies that she's been habitually dishonest.

Er? '[...] so why isn’t she honest with the British people [in saying / and says (or some other way of replacing the infinitive dependent on 'doesn't')] how she plans to retain it' has exactly that meaning (of an implied future action). Yes, 'why isn't she honest with the British people ...' without the rest of the sentence can be seen to mean something different, so perhaps I shouldn't have left out the rest. 'Why doesn't she be' adds nothing except for a faint trace of the strengthening auxiliary verb but only jars (and is quite unnecessary).

This is why linguists test data on native speakers. You're trying to come up with grammatical reasons why the phrase is wrong, when the experimental data doesn't bear this out.

Well, as above, I think you're wrong in postulating a potential for a different meaning here. I have every sympathy with attempts to introduce nuances of meaning that don't quite fit into 'established' grammar, by the way, as ultimately grammar is the servant of logic and not the other way around. However, here I think it's an attempt to export a construction from its established context in a way that doesn't work.

The question of parliamentary language or not is very interesting. It's obviously only speculation in this case, but there is, of course, a strong tendency in English to express things indirectly (e.g., ironically), partly because people had to tug their forelocks most of the time, but also because the grammatical opportunities are there and easily accessible, e.g. conditionality. By contrast, a lot of German native speakers can't do the German Konjunktiv properly, which is undoubtedly partly why it isn't used as much as equivalent forms of expression in English. I wonder to what extent, if any, parliamentary convention shaped general usage, if at all, or whether it's the other way around.