There's a lot of reasons why:

1) Empire is one of the two evil only cabals. Evil only cabals lend themselves to more "evil" behavior, which includes less mercy, greater aggressiveness in PK, use what can be considered "lame" tactics (here's a hint, several of their cabal powers are extremely effective in a gang setting), etc. Scions have less numbers, so Empire has both the "evil" quality as well as a lot of ganging. Also, I think evil characters on average are more likely to sacrifice equipment they can't use (although I have no numbers on this, just a feeling)

2) Flexible restrictions. Just as you'd want to complain (and rightly so) about a battlerager quaffing a haste potion to bash you down, or a tribunal killing you in town when you weren't wanted, people want to complain (and do) when Imperials break the law. However, while the first two are non-negotiable and will certainly get you booted, an Imperial breaking the law can be questionable, good roleplay, bad roleplay, or somewhere in between. Also, they don't always get punished, whereas in the first two cabal cases, the player is more likely to be punished, and harder. This leads to a lot of sour grapes.

3) Bad/mediocre players are usually more outspoken. While the hatred for roving Fortress gangs may be high, the evil players that face them are usually more competent on average. A very strong player is not going to complain about the 2 bard and elf healer combination that ganged him down on QHCF, or at least very rarely. Compare this to the amount of mediocre/poor Fort players that feel the need to post on QHCF every time something bad happens. I can't say that I'm not guilty of complaining myself, but it's usually about specific skills/spells that have an imbalance in certain ways. For what it's worth, when I spot these things, I try make my next character use them to the fullest until they are changed from blatant exploitation.

4) Competitive spirit: There's a natural tendency to hate the person that kills you over and over, making the game less fun for you. Since Imperial players (on average) kill more often and die less often than your average caballed player, they are easy to hate. Hard to "hate" that acolyte bard that you just steamrolled for the 13th time. Even if he complains, you might just think he's lame, but not hate him.

5) Russian influence: There are a lot of Russian players that have been frequenting the Empire cabal for the last two years. If you are an red-blooded American (and a majority of the players who play this game are from the States), I don't have to tell you what this means.