Because the copy is completed prior to WOW Classic TBC Gold release and is released. Classic has all the characters, so there is no copy delay if you select Classic. If you select TBC, the copy could cause downtime (just because it's not instant) and imagine a lot of characters being copied at once; could bring the system down.
The boost will usually become problematic because bots probably use it, creating even more issues with bots. If bots could be controlled, the boost wouldn't be a problem. I have 5 friends returning to classic due to the fact that we all enjoy tbc and they are only joining to boost their game beyond the vanilla content.
Our characters are in Classic. They certainly don't want down time for people going to TBC. The entire database will be copied prior launch. If you select TBC it change the "active" bit on the TBC database to true, and the "active" part on the Classic database will be changed to false. You can then pay $35 to have the Classic bit reversed.
I don't know, but it's Asmongold's livelihood. Asmongold has admitted that he doesn't have any other skill that is marketable. Some people I can understand to buy WOW TBC Classic Gold an extent, especially when it comes to buying these micro macrotransactions. Even though we disagree with the direction of ActiBlizz, a lot of us want to play TBC.