The fire risk is a theoretical one; It's been around since mobile phones became common (I recall it being discussed as a hazard when Petroleum Legislation enforcement was part of my job in the early 90s). Another reason for not using them in petrol stations is that the RF may interfere with pump systems, including the totalisers (as they were not intitally tested for that sort of EMC) - I beleive that has been demonstrated, although no-one will admit it!