I expect you're right, but I don't really understand why. If those interviews have been going on for years, I don't see why the broadcasters didn't just show the politicians up for the fools they are.
I guess because previously they'd use a short clip of the interview on the Six O'clock News and the rest would go in the bin. They didn't have anywhere to broadcast the whole interview (much of which would be boring anyway).
Now though, they put the soundbite in the Six O'clock report and also put the whole thing on the website, where people who are really interested can watch it if they want to.
I think it's also the case, though, that giving short soundbite answers suits journalists as well as politicians. If a politician gave a long, rambling answer to every question that wouldn't be very useful if what you want is a ten second clip to put in your report, so journalists will sometimes encourage them to simplify and shorten their messages.
The Miliband interview clearly takes this to extremes!