I can see how frustrating this would be from the guest's perspective.
From the host's perspective, I might get several inquiries about the same dates (or overlapping dates, in some cases) - everyone seems like good people, and I'd be happy with any of them staying in my home, so they each get a pre-approval.
Guests use this as a way to see what their options are without having their $ locked into a single reservation request (where the host has 24 hrs to respond). Maybe one of the people I pre-approved will book, but maybe none of them do. It would be unfair for other guests (and the host) to have a calendar blocked off on the chance a pre-approved person *might* reserve. What if they send inquiries to 10 hosts in a town? That's half the hosts in my town. That's an awful lot of calendars held up, and artificially removes options for other guests who are trying to find accomodations.
I've had extensive conversations with people who never end up booking, despite their intention to do so "tonight", "tomorrow", "in a couple days" - until somebody has enough interest to put their money down, it's silly to hold dates for anyone. If you're very serious about a place, send through a reservation, not an inquiry.
A reservation request will override any pre-approvals that are already in place. The host would have to decline "a bird in hand" in the hopes that one of their pre-approvals was serious enough to actually book.
Perhaps there is a better solution than what they're currently doing, but holding dates for a pre-approval wouldn't be one of them in my book.