The general population can not be expected to eliminate every allergen. They can restrict and work with the allergic person as best as possible, but no one can reasonably think that they won’t ever encounter a dog at some point.
But up until the last decade or so - whenever people decided that they just couldn't bear to travel without a pet -, most people with dog allergies did NOT, nor expect to, encounter them on airlines. People with allergies are OK with having to take medication because another passenger NEEDS to have a dog with him/her because of a disability or medical condition, and also having to inconvenience themselves and others to find a different seat away from the dog. They do have a problem being forced to take medication and juggle seats because another passenger WANTS to have a pet with him/her. That is what people are talking about - legitimate service animals versus pets that people just want with them.
I don't expect the rest of the world to eliminate allergens, and people with allergies have always avoided places where they know they'll be exposed (Petsmart? Dog Parks, homes of friends who have dogs, etc) and they all know that they may encounter allergens and be forced to take extra medication, but I would also hope that others would have the decency to not expose others to an allergen if they can avoid it. Most of us expect to encounter dogs while walking outside, and in the open air it usually isn't a problem.
The numbers of "service dogs" have mushroomed in the past decade, and people are finding them in more and more places where they never ever encountered them before. Again, if they are true service dogs their owner's needs trump those with allergies. But if they are not , their owners have selfishly decided that the needs and health of others do not matter so long as they can have their pet with them.
Yes, there are constant exposures to allergens - but wouldn't it be nice if people did not make the situation worse and stop doing things that they know might make others physically miserable?