Keeping your word is, IMO, morally virtuous and arbitrarily violating it is not. And, as a matter of fact, a promise made in an advertisement can indeed be legally binding.
There IS IN FACT no promise made.
Just because I advertise lawn mowing does not mean I have to mow YOUR lawn.
I may even make a tentative agreement and an appointed time...
But when I show up at your trailerhouse and see your lawn covered in dog crap and rocks, i am still free to queer the deal and drive off.
You are not entitled to my service.