They do it because it costs them virtually nothing. Similar to email spam, they need one response out of thousands, if not millions, to make it profitable. As for decency, they are happy to inconvenience, annoy and offend the majority of people to make money off one sucker.
It's the same underlying attitude we've seen on the streets: "I'll do what I want regardless of the consequences to anyone else because I don't care about them or society in general."