This has been a matter for discussions before and we have not been consistent. I'll have a chat with Chris to see if we shall straighten things up.
I still find this a hard thing to decide. Some transcriptions stay very close to the original and therefore, in my taste, should go under the original composer. This seems to be the common practice, e.g. Bach-Busoni, Bach-Siloti, etc.... and I sort of regret I wanted to deviate from it.
On the other hand some 'transcriptions' are very free and have so little to do with the original hat they are almost new compositions. I feel that the Siloti reworking of that Bach prelude (which I do not much care for BTW) is not so much a transcription, for which there would have been no need anyway, but a very romantic piece only loosely based on that prelude.
So... the more I think about it the harder it gets to decide. Probably the only correct solution would be to put them in two places. Though I don't feel that Siloti thing belongs under Bach, no more than the Busoni Chaconne does (which is why I put that under Busoni).