Because it would devalue their licence/pension plan.
Because their beer suppliers would probably look askance at it.
Plus, of course, giving away free booze is becoming a major no-no under the current licensing regime, so they would stick you with something if you were doing it in a full, permanent, retail premises. However, at a temporary stall I could see it working.
The Food Safety cert is part of the apple-selling paperwork. A couple of my friends are planning to set up a non-food stall in a Dublin market and they're being stuck with public liability insurance as well.