Blizzard can't do what it pleases. It must abide by the rules of its own EULA and TOS or else it can face law suits from disgruntled customers or whatnot.
They've written up the EULA / TOS in such a way that they can pretty much do anything they want

Strange how they allow the purchase, trade or resale of the actual game itself but not the account. They still won't do anything about it.
They most certainly will - I've seen guildies stripped of purchased accounts while having their existing accounts also banned (same billing info I'd imagine).