It has been said that the accompaniment vs melody rubato -- what I am calling Musical Dissociative Disorder -- is something that is best exemplified in the vocal literature, as I presume, a demonstration of melodic freedom of the singer, etc. However, I would have to say that listening to a performance where the accompanist (pianist or conductor) did not in fact accomodate the accompaniment to the singer's interpretation, thereby maintaining the vertical integrity of the composition and limiting rubato to the tempo as a whole, would be looked upon as a poor accompanist indeed. I'm sure many pianists here have good or even extensive accompanying experience (I do), and the fact that no matter how flexible the soloist or conductor (choral works) can be, that the pianist can "follow" is recognized as the achievment of art and skill -- just plain ensemble ability.
There's a difference between "accomodating the accompaniment to the singer's interpretation" and pedantically following every microscopic nuance of the singer. As a professional accompanist, I want to be sure that I can
follow the singer (or instrumentalist) as closely as I want--but it doesn't always mean I should
. There are times in music when a "soloist" (I hesitate to use that word, but there isn't a better) wants to be able to push against a firm rhythmic structure without it giving way. If the accompanist is too "sensitive", it can cause the performance as a whole to lack conviction.
To be fair, this sort of rubato is more common in popular music, jazz and musical theatre (especially where the composer has set speech rhythms in a reasonably natural way), and also in the virtuoso violin repertoire, than it is in mainstream lieder or art song.
Since you seem absolutely convinced that this sort of rubato is wrong, it's unlikely that I can persuade you otherwise just by typing a few words. But I can tell you firstly that top professionals occasionally do this deliberately, and secondly that you've probably heard it without being aware of it. (If it's obvious
that the pianist isn't following, then they're overdoing it.)