I don't think they should have to. Though if you ask nicely they could give you a bottle of white spot cure - it only costs a few bucks. you shouldnt not have a bottle at home just in case anyway
But did you not look at the fish before you bought them? Not saying they arent necessarily to blame, but when im purchasing, I will watch fish for at least 5 minutes - much longer if the price is over $100, checking its behaviour etc.. - never assume that the person getting fish for you sees everything, as human beings, mistakes can be made.
But in the meantime, do a 50% water change with a gravel vac and bump your temp upto 28C until you get treatment.
and yes, it couldve been developed on the way home - from the stress of being netted, transported, etc..
oh and the wunder white spot cure shouldnt stain your silicone. it will make your water radioactive green for a few hours before disappating (remove carbon before using). the dosage should be 1 drop per litre. then on day 3, its one drop per 2 litres, then on day 7 do a 50% water change. If you have scaleless fish such as loaches and plecos, do half doses (it still works).
Hope this helps!