Yes, kaeru, it's as pat describes. The rationale for it is that it would be tedious to play all the plurals that can be made by simply adding an S, whereas there is a bit of thinking required in coming up with the valid derived words that can be made by adding ES (or ED, etc). It's unusual to have a puzzle with quite so many allowed plurals - a result of having the letters for words ending in SSES and SHES.
I took this approach originally from the rule that is generally followed in newspaper puzzles of the Target Words type, although the stated rules usually just say "no plurals" or "no plurals ending in 's'".