That was my thought as well. It makes no sense to me to put something so unpleasant into a horse’s mouth - if I were the horse in question I would be even busier with my mouth while desperately trying to spit it out!
I have a homebred mare who has a busy mouth; she internalizes stress and that’s where it comes out – she’s always been oral, especially for a mare (and this has been going on since she was a baby - it’s just her!)
After I put her bridle on, I give her a piece of apple (which she loves), and it helps keep her mouth moist – then after mounting up I gave her a couple of sugar cubes. I will also stop mid-ride and give her a sugar if she’s done something especially good. She knows the word “sugar?” (which comes immediately after I warmly say “good girl!” in response to something she does well, so it’s similar to training the R positive “click/treat”), and she will come to a nice square halt and wait to receive her reward. 
She produces nice lipstick and it helps keep her mouth moist and happy – but there is never excessive foam.
I can’t imagine putting anything unpleasant into a horse’s mouth, ever. Doesn’t this just defeat the purpose of promoting a happy, soft mouth and acceptance of the bit?
Also, it’s just cruel. 
