They honestly should get bans if they are knowingly doing this.
It has already been illustrated that top guilds continue to do this and then say there is no other alternative for them to sell the boosts. To be clear, its through middlemen and NOT their own guild affiliated boosts.
Blizzard has only made half measures and now it appears those are mostly pointless. I do see less advertise spam in LFG and Trade BUT it does crop up again. Nobody should be exempt from punishment for boosting with a 3rd party, but unfortunately just like IRL if you are a big name you skirt the rules 
. In addition to that, simply banning the one spamming advertisements does nothing, you have to hurt their supply lines (i.e. the players giving the boosts).
We get that you boost but if its in Trade and I see it I will report it. I may be waiting on a key, on an arena queue, for a friend to log on, whatever the case may be if you are not level 60 nor have the credentials to give what you advertise for then bugger off.
It is against the ToS in certain circumstances, which the OP mentions and the people you are responding to have also mentioned:
Per blizzard support page: "Organizations who offer boosting, matchmaking, escrow, or other non-traditional services, including those offered for gold are prohibited, especially those who operate across multiple realms.
Accounts that are used for the primary purpose of advertising may be closed."
As per what Kaivax said in a CC thread:
That is the problem. So yes what the thread is about is against the ToS.