It's not just a soapbox. The teacher who does this is using his authority to encourage people to listen to and share his religious beliefs. People who wouldn't otherwise sit there and listen to the sermon or pray the prayers do so out of politeness and respect for his position as a martial arts teacher.
I want to reiterate that I am not an advocate of mixing the two. That being said, there is a difference, IMHO, how it is conducted. If there is a dojang down the street that doesn't label itself as anything other than a TKD school, and once you sign up, you realize it is religion + TKD.. . Well that would certainly turn my stomach, and I firmly believe the two have no reason to be mixed together in that type of setting.
However, if a mosque, church, synagogue, etc.. . chooses to incorporate a martial arts program, I see the parallels that CAN be made between the code of ethics, moral culture, and self-discipline that is taught in the martial arts and the proper "spiritual" path most religions preach. IMO, that would probably get in the way of good training, but that's why I don't participate in schools like that.
If a school down the street openly labels itself as "kicks for Jesus" or whatever, so be it, who cares? It's pretty obvious it is a mix of MA and Religion before you even step foot through the doorway.
Again, I want to make clear I do not like the idea of mixing the two, but that's my personal belief. I just don't see it as wrong, as long as people know what they are getting into before they sign up.