They did it because they saw in their work environment echoes of the environment portrayed in the comic, of which Dilbert was as much a part as the PHB.
This seemed so blatantly obvious (and the answer if asked "why did you put this one up?") that I'm agog at anyone thinking otherwise.
He was right about the bad for the morale part.
Similar thing happened with Idiocracy recently...
American corp in Europe for ref. Defence. Absolute top tier stereotype asshats.
Some 'trinkets' are worth more to the employees than they cost the company to provide. So it's rational to provide them. Think of Wally's beloved coffee for an example. Or look at Googlers' lunch.