Actually, I don't think it would be that difficult to make the game check when requesting to start a new game rather than leaving the manage deck area.
However, it's been brought up before that this allows newbs to get into a whole lot of trouble by selling off their cards and not being able to buy enough back to have a 30-card deck. Then they're stuck with less than 30 cards and unable to play any games due to having a deck that's too small, which means they can't make money to buy more cards.
It can be annoying, especially when you have a slow connection which makes adding and subtracting lots of cards a lot more time consuming and frustrating than you'd imagine (for me, it's click 30 times, then wait 10 minutes while flash freezes up trying to load the cards) -- and when you're just starting out with very few cards, that juggling act can get pretty complicated. But I can certainly understand the reasoning behind having that protection there, and it's not really that problematic if you only need a couple of cards, anyway.
