There are a few reasons. Firstly, I've done it before. Twice, actually.
Secondly. @stilgherrian did it this morning:
Mostly, however, what I've realised is that most people don't really care what people they have never heard of say on twitter.
Unless you're employed in politics or, like me, you have what probably amounts to an unhealthy interest in it, you probably couldn't name five members of the upper house. Heck, I'd have to think about it.
O'Farrell knows this. Either he doesn't care what Phelps does on twitter, or he knows it doesn't matter. I suppose whilst the second is true, the first probably is as well.
The interesting thing is that, as I understand it, Phelps may actually be the most genuine NSW politician on twitter - this is apparently exactly what he is like in real life. If you want, you can go read what he had to say yesterday at about 4pm and why he said it.
He saw an (admittedly pretty stupid) comment from a leftie, and decided to get stuck in.
Phelps likes running about on twitter, making trouble, and brawling with people who annoy him. He even took the chance yesterday to mock someone for being unemployed, although in fairness that someone did bring it on by calling him a "knob end".
Quite the place, twitter.
In any event, this, apparently, is the NSW upper house. Given that the voters don't seem to care either way, and O'Farrell certainly doesn't, I suppose we're stuck with it.