IMO it's a bit of a regression due to a commit Campbell made during bcon4 IIRC. (https://developer.blender.org/rB6cc5bdc99e63b05248f132833bfe0259c2a27923)

The problem is that we have a bit of a tradeoff here.
IIRC, Campbell's commit disabled scaling and/or rotating so that handles don't go crazy when you simply select everything and scale up/down a whole cluster of stuff. The alternative is what used to be in place, which intended to allow this sort of modification when you've got just a single keyframe + its handles selected (AND with the "individual centers" pivot mode active, as in this file).

Gentlemen, do let me point out that this behavior is not only applicable to a single selected keyframe.

If the pivot form 'Individual Centers' is selected then this is what is needed to scale the tension on multiple selections at one time.

Oh, and I forgot to mention this...

This functionality is indispensible when it is necessary to vary the tension on multiple, equal curves at once.

A particular example of this would be when varying the tension a keyframe of an object's scale animation - equally on X, Y and Z - to constantly maintain the object's proportions.