Some of the comments here make it sound as though as soon as a business announces they are dog friendly, every single dog owner will go there - this is not the case, there may be a slight increase in those people who take their dogs inside but I doubt it will be a considerable increase.
For those who have allergies, the service dog anecdote still applies, you need to be careful, for some it does not matter if their allergy is from one dog or many dogs, sadly the onus is on you to mitigate.
There are some who treat their dogs as children, there are some who would not - I would not leave my dogs outside a shop just as I would not leave them in a car if the weather was warm or hot. There are times when I will have my dogs with me as it is more convenient to take them with me to walk them, and do other things I need doing. I have a lockable cage in the car, so can leave the door open to ensure they do not get too hot, but if I have to park further away from where I am then even with the locks I will not leave them, in that instance I am grateful I can take them with me and go into businesses - it is peace of mind for me.
The weather in the UK can change very quickly, if it is too hot I will leave the dogs at home, but like today it was very cold this morning, but now it is very warm, too warm to leave dogs in a car unless somewhere close to where I need to go.
There are people who are anti-dog, there are people who tolerate dogs, there are people who like dogs but do not want them inside and of course there are the very entitled people who want their dogs to go everywhere, but to say it again, it is the business who decides their dog policy and they will monitor the effect it has on their business. They may also take notice of correspondance sent to the for and against the policy - it really is as simple as that.
If you do not like a business which allows dogs then do not support them.