I *don't* understand singling out 'offensive' material.

Because, in a corporate situation, the difference between "non-work-related" and "offensive" is the difference between a nuisance productivity loss and a major lawsuit.

If an employee is reading online Dilbert cartoons on company time, and a co-worker walks by, it's not a big deal. But if an employee is viewing pornography and the same co-worker walks by, the company can be sued for sexual harassment.

It's a similar situation with e-mails. If you use the company e-mail system to send jokes to your co-workers, and a sexually explicit joke gets forwarded to the wrong person, Game Over. There are actual legal precedents in this area.