If the town chooses to humanely euthanize, the HSUS won't stand against it as its a humane method and a choice of the town. There's no legal reason for them to step in. Spaying and neutering feral cats doesn't solve the problem of existing feral cats. Not only that, but they may not have the funding for spaying and neutering all of the cats. So long as the cats are caught humanely and treated so until the end then they aren't doing anything wrong.
No. S/N does solve the problem, whereas trap and remove does NOT. If this town were to trap and kill cats (and no, there's nothing HUMANE about it - and it is also not euthanization, it is killing. Euthanizing is what you do to a terminally ill patient, not to a healthy cat who happens to live outside) they would miss a couple, and those would breed and begin the problem all over again. Or cats from nearby areas would move in to the newly created vacant space.

On the contrary, if the cat population is stabilized through TNR, the resident fixed cats hold the territory while the population gradually diminishes.

I've seen it time and again - the neighborhoods where people insist on killing the cats, those are the problem areas year after year after year. New kittens every few months. The killing goes on and on.

S/N is also a heck of a lot cheaper than catch, hold, and kill. The blue solution ain't cheap, especially when you have to keep doing it over and over again because all you've accomplished is allowing cats to continue breeding. It's also easier to fundraise money for TNR than for killing.

HSUS has been very lazy about coming to terms with TNR. They're finally publicly backing TNR as the humane solution to feral cats, years after they should have, and this this guy goes ahead and says something like this? That's exactly the kind of reason why my pleas for money from HSUS go right in the trash. I'll send my money to Alley Cat Allies and other groups that support TNR with no ifs, ands, or buts, thank you.