Mielr said:

But why combine it with the ROTJ ewok/celebration music? Just to be able to squeeze both unreleased pieces onto the disc? (In which case I've answered my own question). ;-)

Yes.

Keep in mind that at the time, soundtrack albums weren't the archival-style presentations that we're used to today. They were abridged and re-arranged for listening purposes. The Anthology box set still reflected this approach, even as it added a treasure trove of additional music (and is still the best-sounding presentation of music from the trilogy on CD — the RCA/Sony Special Editions have awful sound on Empire and Jedi). Combining the celebration with the unreleased end credits on Empire was their solution for including a piece of music that would have an abrupt end, and one that would have an abrupt beginning without wasting too much disc space.