The problem with that is that the people who do this kind of thing unthinkingly - leaving perishables that need to be kept in the fridge, unsolicited and unannounced out overnight on somebody's doorstep - are the same kind of people who, at very best, would read the first six words of the message and no more, and would thus then keep on doing it with the cockles of their hearts unwittingly duly warmed.
The first rule of signs is that the only people who ever take the trouble to read them are the people for whom they were never required in the first place - and who then scratch their heads in puzzlement at why anybody could possibly need a sign to tell them something so stinking obvious as that.
The thing is that, although they will be running a tight ship and abiding by all of the rules and safety regulations, it's a bit misleading to call them 'professional', when all/most of their workers are volunteers.
These are extremely kind people who are giving up their own time, energy and probably indirectly money as well, in order to do something amazing and helpful in the community. It's one thing that the vast majority of the people who live in the community and who would potentially have time to volunteer to help don't do so; but for them to be doing something so obviously stupid to thwart and add unnecessarily to the workload of the very tiny minority of people who DO volunteer and give so much of themselves is actually quite a nasty thing to do, really.
Like the charity shop fly-tippers, we like to assume that people have good intentions and are just foolish and unthinking; but there are plenty of people out there who would quite knowingly and unashamedly seize on 'doing a good thing' as an ideal excuse to save themselves some time and/or money and space in their own bins. As long as they're alright, that's all that matters.