fascination,
In the case of a new hairdresser who was scheduled in advance of a competition, this could mean not having your hair done and having no time to find someone else. If you can do your own quickly, that might not be a huge problem. But generally, those who schedule a hair appointment with the competition hairdresser feel they can't do their own hair adequately. That's why the scheduled.
So it is inconsiderate for the person selling this service to force essentially captive clients to be their audience.
It's really not the same as someone merely putting a sign in their yard, posting something of their facebook page or doing something that others would see but can decided to ignore. It's not even like a hairdresser plastering her own shop with signs. Usually, the need isn't as urgent in normal life so any new client who found you on the web, phoned in an appointment and was shocked can likely just turn around when they see they've arrived at what looks like political party headquarters!
I'm sure that hair dresser found that her choice of attire affected repeat business at comps, though perhaps not as much at it would in real life. (I don't know how many people pick a competition hair dresser based on reputation). It may also have impacted tips alot. A hair dresser with the opposite point of view would also find they got less repeat business. The magnitude of the business impact would be affected by the strength of feeling on the part of clients.
It's definitely not a good business practice in sizeable part because it's not fair to treat captive-audience money paying clients this way and objecting clients will see it that way.