I agree with you, Scotsman. In fact, often on commercial television in the U.S., we are treated to a whole page of disclaimers prior to the showing of a theatrical film: "This film has been edited for content, edited for time, reformatted to fit your screen [how do they know the shape of my screen?] and time compressed". Time compression is the worst offender; a bit like PAL 4% speedup, it is the deliberate speeding up of the film to fill a shorter period of time - characters start sounding like Mickey Mouse! And I won't even get into the insane use of promos for alternate programming that constantly takeover the lower half of the screen, obscuring the program one is trying to watch.
Fortunately, non-commercial cable channels (Thank you, TCM) tend to show films unedited and in the proper aspect ratio (although I'm troubled by HBO and SHOWTIME habitually showing certain 2.35:1 ratio films in 1.85:1, cutting off the edges of the composition in the same way 4:3 television used to do). The most ludicrous experience I had with commercial television happened over 25 years ago when a local network affiliate broadcast CITIZEN KANE in a two-hour prime-time block. Not only was over thirty minutes of the film removed to accomodate commercials (which made it difficult to recognize the lead character as a newspaper publisher for the first hour), but the station cut away to other network programming before the sled could be revealed in the furnace at the film's end!
As to the changes to the Arena documentary in the U.S., it was broadcast by the TNT cable network as ORSON WELLES: A LIFE IN FILM in what I believe was a three-hour block. To fit in the requisite number of commercials, a sizable chunk of the BBC footage needed to be removed.