According to Phra Cittasamvaro, it’s because Thai monks are, after all, mere mortals.

In fact monks are like any other Thai – they have opinions too. The reason Thailand keeps them away from politics is to stop popular teaching monks using their ‘moral credentials’ to sway voters for one party or another. It is probably a good idea.

[…]

Monks feel that even if they should not be involved in party politics, they are quite free to take a moral stance, which is why so many have joined the Red Shirt rally over the last few weeks. Naturally, where the line is drawn between politics and morality is very flexible…

I encourage you to read his thoughts in full. For the perspective of a couple academics on this issue, check out Danny Fisher’s interview on Shambhala SunSpace.