I agree. These things should be clear beforehand. If you want to charge, change your boarding agreement for future clients. Not much you can do for your current situation.
This. ^^ Plus, from what you are saying you are not consistent in who you charge for what. You didn't charge for bringing in a new horse, but you want to charge for this 40 minutes of walking a horse that you use as much as the owner? Really makes no sense.
IMO you need to make a policy and stick to it. It does NOT have to be just for new boraders. You can give it to your current boarders in writing, amend their agreements, and be done with it.
I have boarded a lot of places over the years, and there are very , very few that would have charged me for this. However-the one that would had a full page long price list....covered most things, and was part of the boarding agreement. It was a bit intimidating, including the late fees for board and the fact that after about a week late-they own your horse.
Mostly I have (and prefer to) board at places where everyone is there for the horses and helps each other out. Just my preference.
Right now one of mine is with a well respected reining trainer. My friends horse just came back from another one. Neither one would even consider charging for this. Horse is groomed, clipped, bathed, in a heated stall, indoor arena and ridden an minimum of 4 times a week, for only $300 more than what I paid at the place that had "the list". You tell me where I would rather have my horse. And I now have someone to help me every time I ride my horse. Lessons EVERY TIME included! (One lesson at the other place was $55) No comparison.