And/or the broadcasters know that some TV's let you zoom in on a letterboxed image. In both cases the broadcasters want their bugs on the screen.
It's usually for STB's that are set to 4:3 centre cutout.
If only the stations could deliberately add the bugs and scoreboards separately on their high-def outputs and their standard-def outputs. But for some reason, they don't. They just get lazy and cheap and compose the shot once, with all elements in place, then slice of the sides and downconvert.
In terms of framing shots. You can only compose one shot with one camera, so you couldn't do both unless you had 2 sets of cameras, 2 directors and 2 scanners. Then you'd still need 2 sets of character generators and 2 different uplinks. The chances are they're already uplinking 2 feeds (main & reserve) so they'd actually have to uplink 4 carriers, very expensive. Give it a few years until 4:3 sets are firmly in the minority and then the situation may change.
When we moved our shows to widescreen it took our cameramen a few weeks to get used to the extra screen estate that widescreen brings, that said they still frame for 4:3.
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