HBO and the other "premium" cable channels also do this. Not every single time, but much of the time. Try watching any of the Lord of the Rings movies on HBO or Cinemax for example. They're all 16:9, even though they were 2.39:1 "scope" in the theater.[1]
Forcing everything to be 16:9 has become the new "pan and scan", and it's actually been around for a while.
[1] Some of the time a movie has been filmed in a format which contained extra space on the negative, such as "Super 35", and in some cases the 16:9 might actually be showing more of the image rather than less, but it's very hit-and-miss and requires a custom transfer and master of the movie (which HBO has been known to do in at least some cases).
Forcing everything to be 16:9 has become the new "pan and scan", and it's actually been around for a while.
[1] Some of the time a movie has been filmed in a format which contained extra space on the negative, such as "Super 35", and in some cases the 16:9 might actually be showing more of the image rather than less, but it's very hit-and-miss and requires a custom transfer and master of the movie (which HBO has been known to do in at least some cases).