unfortunately it is just too much FUN to bash bush. He is just such an easy target that it is always really hard to resist.
Politics these days is almost entirely about personalities and appearance and very little about issues or policies... so when you see politicians and commentators "bashing bush" or "bashing kerry" all the time it is only natural that you tend to follow suit (from whichever side you start on)
It should also be noted that it is much easier to campaign and vote AGAINST something than for it. And as Bush is the current president he has a much higher profile than Kerry and so is an easier target.
As far as ican see it is largely down to the "with us or against us" stance. Anyone who criticises bush seems to be branded as "liberal" and therefore unamerican and not worth listening to. When someone tells you that it is quite likely that you are going to respond on a less intelectual level and "bash bush" as your reasoned arguements have falled on deaf ears and you need to try something less subtle.
It is also the case that a number of people on these forums are from outside america, and bush is universally seen as an idiot (perhaps even to an unfair degree) outside of america.
bush meets arnie