When someone cancels a hotel room at the last minute in a small place like Symi it can really hurt as the hotels are small and there is no passing trade so the chances are high that room will remain empty. Losing a night in a hotel with 100 rooms or with all year round trade is a drop in the ocean. Losing a night in a hotel with 10-30 rooms is a significant dent in the hotel's income, particularly as the season is getting shorter and shorter due to lack of boats in April, May, early June and October.
By the way, the cheapest way to send money from the UK to Greece is through Royal Mail as they don't charge anything for the transfer. I can't remember now what the minimum amount is but a lot of our UK clients use that, or Transferwise. As we don't have card facilities at SVA all our payments are by bank transfer