Single-eng taxi in is SOP in my airline - assuming adequate engine cool-down time, no adverse taxiway/apron characteristics etc etc. Absolutely no problem with handling, as long as you anticipate any taxiway gradient. Saves in the region of $millions if applied over the whole (large) fleet.
Screwballs, I would be very careful of using the words without doubt in this industry; no matter how sure we are of something, we are still prone to error.
Down the back of a 737NG is not the best position to judge things.
Why not write to airline concerned, and ask for an explanation of their policy in this regard? Or file a quick CHIRP report, which can be followed up with the airline whilst preserving your anonymity.