Why do Inferno towers turn off?

Why do Inferno towers turn off? Xbows, infernos, eagles, and scattershots all have a finite amount of fuel which is instantly refreshed every time you log on (just like how traps are re armed). If those defenses attack for long enough without the owner logging in, they will run out and stop functioning.

Xbows, infernos, eagles, and scattershots all have a finite amount of fuel which is instantly refreshed every time you log on (just like how traps are re armed). If those defenses attack for long enough without the owner logging in, they will run out and stop functioning.
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