and welcome to J.SE.
It's certainly a wrong thing to do, but the Torah's prohibition per se was focused on "creative labor" (the Hebrew melacha which corresponds to the Greek technos). The point was to take a day off from building/improving the world. Burning charcoal to cast metal is building/improving. Burning something down just out of spite is not "building/improving" the world, and was thus not included in the direct Sabbath prohibition per se.
The rabbis of the Second Temple era did, understandably, realize that people could run amok doing very odd things based on this definition, and prohibited "destructive" behavior as well. But "liable" as used in the Talmud's language means "violations of a Biblical nature", which are more serious.
Of course if someone destructively burned their neighbor's property on Sabbath, they've violated interpersonal commandments, just as if they did on a Tuesday -- but that gets discussed in a different part of the Talmud. (Tractate Bava Kamma covers torts.) The phrases you are citing are focused purely on the technical definition of "creative labor" vis-a-vis the Sabbath prohibition.
