It makes me sad that P!P is so quick to ban cards that are bad for the format from the professor cup, but not from OP...perhaps in the future P!P will consider bans or limits on cards that curtail the format to much.
Although there are cards in modified that we might not like or we might think ruin the format (not that everyone ever agrees on this), the fact is that modified is pretty much designed to be the way it is.
The Professor Cup format is made out of modified. It was never part of the orginal design and isn't thought through in the same way, so there is a much greater chance of a card sneaking in that completely unbalances it. The European Prof Cup pretty much showed that as long as HTL was legal, there was nothing else worth running than Whirlipede. We have never had a situation that extreme in the TCG. Not even in GG and Luxchomp days.
So it makes sense to ban HTL to preserve a fun, one-off format. Especially as there are none of the issues that you would get with a modified ban (unwanted precedent, people angry because cards they expected to be able to play were banned).
I guess if we ever had a 'play this or you will 100% lose' in modified, then TPCI would act . . . like they did with the early rotation when Sabledonk became ridiculous.
In general, I'm opposed to modified bans because there will never be a consensus on what needs banning. You always get a ton of people shouting for a card to be banned for any number of reasons: Absol SW for making lucky T1s too influential; Claydol GE for 'killing originality'; Cyrus's Conspiracy for making SP overpowered; Catcher because it hurts set up decks; Tropical Beach (or Catcher, or Claydol) because it's too expensive . . .
Who decides when you give in to any of these demands? Should any of them have been listened to? Not in my opinion.