Nowt for owt!
For comparison's sake, I do a few days at a local flying club and they prefer payment by debit card. The reason is that it's quick and easy, there's a paper trail, an instant receipt and students aren't expected to wander around with wads of cash.
If you are looking at payment methods from a business point of view, it is important to note that cash transactions also carry a processing cost. Cash is easily stolen, it can be forged and somebody has to be paid to physically go to a bank in order to pay it in - so cash is far from being 'cost free'.
It could be that consumer culture is different in xrayalpha's part of the world, but where I live I would expect a company to accept debit card payments, especially for the sort of money involved in flight training. There's also a consumer 'credibility factor' associated with card payment facilities being made available. Personally I would find it rather odd, inconvenient and perhaps a little suspicious if a high value service was being offered on a 'cash only' basis.
Surcharging (or perhaps supercharging would be a better word for the loco airlines) cards at the point of sale leaves the same bad taste in the mouth as those companies who quote ex VAT prices, hidden charges will always leave consumers feeling 'ripped off' and damages the seller's reputation.