I would of course help someone who needed help, but I think in the US almost anything can be a service dog and they aren't actually always well trained.
I came into contact with an American lady in Germany where I live now who was fighting to get her banned (here) bull breed "service dogs" allowed into the country. They were autism support dogs but she simply relied on their moral support, they weren't actually trained to "do" anything special. So by most people's definitions they were pets, not support dogs at all!
Of course she may have been a maverick and atypical of a lesser known type of support dog owner.
If it wasn't a guide dog I wouldn't assume a random dog in a high visibility vest was alerting me to an owner having a seizure at all, because it's totally outside my experience. It depends just how the dog was behaving I suppose.
Also quite aside from being terrified, allergic or indeed equally in difficulty yourself, not everyone can drop everything and follow random dogs on the off chance it isn't just a runaway or unattended dog and is summoning help for its owner. If it's not clear then people are going to struggle to leave or drag along small children, dependent adults in their care, or cut short a business phone call or abandon their errands or be late to work to follow a dog.
If the dog was very highly trained and wearing a clearly marked jacket saying (if I am alone please follow me, my owner is epileptic and needs help" people might be more likely to, but then the urban myths about criminal gangs using service dogs to lure people into dangerous situations would spring up...
Perhaps dogs aren't the solution.