Why does Mythbusters go to such lengths to avoid showing brand names? They even blur or cover the labels on food items and household products. Is it standard Discovery Channel policy, and if so, why are car brand names and business names (junkyards and stores) exempt?
It’s likely a liability issue. Nobody wants to get sued by a bug-bomb company after you show their product can blow up a house. 
Dunno about the Corvette, but it was such an intrinsic part of the “Stinky Dead Pig” myth, they probably had no choice.
As for junkyards & such, those are local businesses which are probably very happy with the free advertising.
Hmm, I guess that’s reasonable. It’s hard to imagine a beer company suing them for showing that their beer cannot remove skunk odor, but I guess you never know. 
The NHK (Japan’s national TV station) prohibits ads and I’ve seen similar practice of covering brand names (though not so often these days), which was why I wondered if something similar was going on. But Occam’s Razor seems to favor your explanation.
To avoid giving those products free advertisement.
Marc