What I have gathered from US law is that they have to do it in order to protect their trademark. If they don't sue, it's same as giving up the trademark. Please correct if I'm wrong.
If Apple's trademark was restricted to the context of computing (likely), then suing anyone who dared using an Appple in their logo was beyond the score of necessary protection.
One problem Apple computer had is that it's trademark was limited to computing. That caused problems when they branched out to other markets such as 'computers that can do MIDI'
In particular, they had quite a few disagreements with Apple Records about doing audio. Even though more or less the only people who know Apple Corps/Records think "of the Beatles' records", they took action against Apple even as early as 1978 (http://en.wikipedia.org/wiki/Apple_Corps_v_Apple_Computer)
I think those lawsuits may have affected company culture at Apple Computer, now Apple Inc.
Apple's lawyers have always been busy. In the "PC era", Apple sued businesses that dared to use an apple as its logo.