I will stipulate one thing. I never drove a horse so I can't speak with any authority. However, whether or not it hurts the horse, the horse notices it so they feel it. Imagine getting poked in your knee eight times in a row. Most likely you would find it annoying; I dare say while it is not 'painful', it probably is annoying to the horse which is why they respond to it.
But there is another issue involved, the aesthetics of it. I will admit, many gamblers could care less about a horse being booted if they are collecting on a ticket because of it,. However, it looks bad; Bad to the first timer who comes to the track (yes, there are a few of them), bad to the person who refuses to go to the races because of the perceived abuse in the sport, bad to the person who would be happy to see horse racing disappear. In this age of recognizing animals feel like us and 'think' on some level, this visual is simply not acceptable.
I do agree with David Miller that booting does not equate kicking so the penalties should be the same. Hence, I would suggest the racing commissions make a new rule, breaking out booting from kicking and then decide what the fine should be. The fines should be less than what kicking entails.
Other sports worry about image and discipline their participants for doing things which hurt their image. Like it or not, the driver is the most visible person in standardbred racing and in effect, are our spokesmen. Drivers need to put their best foot forward when on the track..