It clearly sounds like a pissed off owner whose had to face one too many people freaking out about dogs being about.
When you have had to deal with customers freaking out, you'll understand why she would have put out an intentionally pro-dog sign.
It sounds like a damage control to stop non dog people coming in and a way to get attention/free publicity from people talking about it.
It is hard sometimes to know which places are out aren't dog friendly so my guess is this was intended to be over the top.
Could it be done in a way less aggressive, yes but would that get them so much attention, nope.
I'm not a fan of aggressive but I know how this will have come out of pure frustration. Sounds like she has a dog she brings in to the cafe and people have complained.
A dog issue arose at a shop in Brighton. Manager brought in their dog who was behind a barrier in the window. Mum and two daughters come in. Daughters stroke and day hello to the dog. Next day manager is called up and told that they had a complaint about the dog jumping up and scaring /hurting the daughters. Despite cctv footage proving this lady was lying as the dog just stayed lying down and they girls went to it but she was clearly nervous and keeping her distance, she put on a complaint in a way to get the dog removed from the store.
Again, dog was in the window, behind a barrier and the window display had been created to include the dog, it was obvious that the shop was dog friendly.
Yes some dog owners can be crazy but so can anti dog people.
I grew up with dogs and love them but you have an awful behaved dog who almost knocks my toddler into a ditch being "friendly" or letting them run around a kids park off the lead trying to bite little kids or other dogs and making her terrified of dogs for months... Yea I'm going to go mad.
Sign worked though. Mumsnet has it as a popular thread with loads asking now where it is and she will stop non dog types going on and complaining.