There are sites that trap you deliberately, but they tend to be on the shady side. As everybody points out, it's annoying as hell, and you have to have an objective that you think is more important than the consequences of alienating people. As Eri speculates, sites that deliberately trap you are often trying to maximize their chances of dropping a nasty infection on your computer. Slightly more benign, though still sleazy, you have operations like porn sites that know the odds of making a sale are remote, when there's all that free stuff available; so they want to maximize the odds through repetition.

But then, in the majority of cases, I think, are all the sites whose navigation is just incompatible with the linear assumption of browser history. If where you want to go can always be expressed in terms of forward and back, fine. But add frames and iframes, and Javascript that does tricky things to a page that sometimes the browser records as navigation, and sometimes not...and it can just happen that you get trapped at some Web sites.

I've always had a problem with my design ambitions colliding with the browser's limitations, and I've built a lot of sites that didn't play well with forward and back. How soon y'all forget...remember the last Zone design? When you changed forums, Javascript switched three frames, which the browser obligingly recorded as three steps. I'll bet that over the years you trained yourself to never use forward and back, and now it seems natural.

It's not going to change. Linear navigation will always be available as the lowest-common denominator, but as sites get more complicated, you really need to reconcile yourself to cooperating with the navigation as designed. It's always well-intentioned, and often works to provide easier nav than the mental gymnastics required to hold a site navigational tree in your head and bumping around forward and back like a rat in a tree-shaped maze.

Don't fight it. Trust the navigation. The navigation is your friend.