Why does the taser encourage misuse?

The taser, provided it works as advertised, provides the officer a sense of absolute power over the victim. Press a button and down he goes. In the absence of extensive ethics-based, legal-based, morality-based training, with some weak-minded and/or overly-excited officers, the following might happen (perhaps you've heard of it...):

Absolute power corrupts absolutely.

Annual refresher training on 'Use of Force' is an excellent idea.