The behavior doesn't exactly switch on pipe detection though. The progress bar consistently shows when outputting to a file.

  $ curl news.ycombinator.com -o /dev/null
    % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                   Dload  Upload   Total   Spent    Left  Speed
  100   178    0   178    0     0    948      0 --:--:-- --:--:-- --:--:--   951

I mean, it's effectively doing tty detection, but I think it's fair to argue a semantic difference, and it's very similar to how ls strips colors and disables listing format when piped, which is mentioned by the next paragraph in the coding standards.

Compatibility requires certain programs to depend on the type of output device. It would be disastrous if ls or sh did not do so in the way all users expect. In some of these cases, we supplement the program with a preferred alternate version that does not depend on the output device type. For example, we provide a dir program much like ls except that its default output format is always multi-column format.