since if you take their souls, you won't get any experience anyway.
This is also consistent with the behavior if you enslave a monster's soul, and then confuse it; if it e.g. steps in deep water and drowns, you'll get a normal kill with experience. Seeing as it's difficult to manually move a newly created abomination or spectral thing to the nearest habitable ground if it's not there initially, and the potential oddities if there is no habitable ground nearby, I'm choosing to interpret the behavior as a feature. As for an in-game explanation, you can only bind enslaved souls to you when you're near them, and if they're e.g. at the bottom of deep water, they're not near you.
git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@8312 c06c8d41-db1a-0410-9941-cceddc491573
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