I think Rob missed a good bet by not giving Queen Bea a rhymemancer named Maynard, but I guess references to her "having the thickness" would probably have hit a fairly small target audience.
All I could think about while I was reading that update* was geeky pawns with mirror-shades rocking out on a chess board. Is that wrong?
As to why she did it, she did it for the same basic reason as it's happened any number of times in history. Scorched/salted earth is worth less than arable land, and knowing nothing she could do would weaken the oncoming force so much as abdication and disbanding, she did her best to deny resources to her enemy.
It's not always a good idea, but against a threat like Wanda, which has overwhelming force and where it can be assumed that any forces left out will be mopped by high-speed dragon sorties, with any forces left in the city destroyed by the Glom, scorched earth (or rather, scorched Erf) makes sense.
*what can I say, her name has a soundtrack. For me, at least.