Most UK petrol stations display a "no mobile phones" warning. Until the advent of mobile phones, nobody thought about it.
I was using amateur radio mobile equipment from the 1960s on, and never had a warning about RF causing explosions till cellphones appeared on the scene.
Urban myth, with a remote possibility - a strong enough RF field could generate a spark, but it would need to be a pretty strong field. I doubt a cellphone could do it.
But in this day and age, regulation is the "norm". Health and Safety, Litigation, and petty bureaucracy have a natural tendency to proliferate. Smile, and ignore the daft rules, that's what I do.