Um, Lesley, that's because you are exercising poor judgement. On the snow thing, you are dealing with a bum, not with a contractor.
There is no reason to pay someone up front for shovelling your snow. We have people who shovel for our houses -- we settle up every couple of snowfalls.
There are some areas where I would be happy to pay up front --- I've just ordered mirrors for a dance studio, 50% up front; custom cabinetry, 50% up front.
As a homeowner, I would be happy to pay a deposit on things like new windows, a new furnace or a/c unit, or on any custom order. It is a matter of judgement. (Home Depot's policy of requiring payment in full for goods plus installation in advance, as an example, sucks big time. Only a fool would go for that, and judging by home depot's cash flow, there are a lot of fools around.)
I'm a well established remodelling contractor. I generally require a ten percent deposit on jobs over ten thousand dollars; under that, I don't bother with deposits.
On the larger jobs, the deposit does little to protect me. I have to pay framers, electricians, plumbers, HVAC, boarders, tapers, finishing carpenters, painters and so on ... plus the materials whether or not you pay me. Their contract is not with you, it is with me.
So ... I do trust homeowners. If I feel at all uncertain of them I politely decline the job.
In your case, I would make no further investment -- emotional or financial -- in the situation. Buy a snowblower, hire a high school kid, whatever. What's gone is gone. Focus on what's ahead.