How can thieves set a plastic fuel tank full of gas on fire, and still fail to completely destroy a bike?

the thieves has tried to set it on fire, possibly because they couldn't get it started despite chiseling the ignition off to get at the wires. So, maybe they tried to burn it to destroy evidence that could lead to their arrest? Like fingerprints?

http://mike-thistledown.blogspot.dk/2013/06/stolenrecovered.html