i cant be 100 percent on this put it could be either to fit in with the time slot...ie say friends is 21mins long and the time slot they have is half a hour but they have more than 9 mins of adverts to show they may cut bits of the show.
Or the more obvious reason is that a lot of the friends and scrubs dvd's i have are certified for age 12 and above, if there is something that is not suitable to be showen at a certain time of day then they have to cut it. I have noticed on E4 that they muted someone on friendfs saying p!55 because it was before the watershed.