My understanding is generally that you can get out of it just before it starts playing (by pressing the menu button, I think?) but once it starts playing it uses the same flag for the anti-piracy warning (or something like that), which prevents getting out of it. This issue does not appear on computers, of course, only DVD players.
Of course, this could have changed in newer DVDs or DVD players; though I haven't encountered any disk which has absolutely no way of getting out of the previews, but I tend not to watch many movies.
Also, I think that I have to point out that you are entirely free to mute the DVD and go off somewhere else until the previews are done, if you're unable to find any other way out of them; "FORCES" is a bit of a strong word, given that you were not (I assume) physically prevented from leaving. This is, of course, not an acceptable solution, but it does eliminate the discomfort of sitting through previews.