If, as another poster mentioned, the stairs are metal, narrow, steep, wobbly, etc, then the safety problem is with the stairs, regardless of what is being carried on them. This should certainly be fixed, but has nothing to do with the buggy issue
Well said Gibon2, you took the words from my fingers. This clarification about the wobbly stairs was actually a very useful addition to the thread because it explains the reluctance to carry buggies but at the same time reveals that the real problem is inadequate staircase design rules.
And I agree about clear policy. In my experience it seems to vary not only from airline to airline but also day to day, depending on who you ask. Hazy policy is annoying for the customer and also very frustrating for the customer facing staff who have to explain it every time.