If I buy the last sandwich, then I've deprived someone else of that sandwich by purchasing it. In case you haven't noticed, seats on an aircraft are a limited resource. You make a "reservation" to declare that you intend to occupy that seat, and the corollary is that nobody else will be able to occupy it. If enough fraudsters reserve seats without using them, the airline has another problem. Say they loaded the galley with 100 peanut packets because 100 passengers promised to board, but now there's only 20 on the flight, what are we gonna do with all these peanuts?
As I mentioned, airlines have plenty of policies to deal with reserved but unused seats, but you've still intentionally taken away a finite resource because you wanted to save a couple bucks.
As I mentioned, airlines have plenty of policies to deal with reserved but unused seats, but you've still intentionally taken away a finite resource because you wanted to save a couple bucks.