Kostya,
I don't have any stake in this situation, as so far I am lucky not to be forced to use outpook [sic!]. (But that might change at some point.)
But I was curious about this case, and I was trying to evaluate this situation. I perfectly understand the logic you have behind the app's behavior.
However, I think, both the ability to choose a different calendar (which is implemented) and the ability to choose a different default/preferred (or least to have the last one used as the preferred) calendar is important. And if I understood the situation correctly, in case of the matching calendar-e-mail account, it is the dead preference.
I do use multiple google calendars most of which are under one account, and I also have multiple google e-mails (for different functions). And I would want most events to go to my "main" calendar, that is "non-matching".
So, I'd suggest that the logic covering what should be the default account is fine, but in addition to that, there should be an option to override that.