You are grossly exaggerating the danger.
If
WalMart were in danger of esploding due to nearby smokers, they would not have the tanks where they are. WalMart would not want the potential legal risk and the city would not allow the danger.
Some time when you have a minute, go to your propane grill and barely turn on the gas to simulate a leaky tank. Next, stand next to the grill and light a match. Nothing will explode because 1) there is too small amount of gas leaking and 2) the air/gas mix is wrong.
The same thing goes with those tanks at WalMart. If there were a leaky tank (which would defy the tanks engineering, but lets ignore that) the leak would either be large, in which case the gas is long gone and therefore could not ignite or the leak is crazy small, in which case it would not create a risk of explosion.
Finally, does anyone remember the mythbusters episode involving smoking at the gas pumps? they tossed a lit cigarette into a puddle of gas. Nothing happened because the air/gas ratio was off.