What is even the motivation for stealing a bicycle seat? How much can you hock it for?
I think I would forgive it if some poor soul had stood here after glancing around furtively to confirm that the train cops were absent, and computed: "Taking this seat will ruin the evening commute of some rich person, and likely the eight commutes after that, but he (probably he) can take it, whereas I need to sell this seat to get the money to buy the drugs to sell the drugs to get the money to buy the bread to feed my starving family—clearly a Kaldor–Hicks improvement, and after my drug-dealing startup takes off, I'll be sure to spend the occasional Monday evening replacing missing seats at a train station bike rack not unlike this one."
Unfortunately, this is probably not the motivation. The motivation is much more likely to be, "Taking this seat will ruin the evening commute of some rich person. What a great idea!"
Remember being a teenager or college student and having someone suggest it would be totally hilarious if you... .
Never attribute to malice what you can to adolescent stupidity.
I guess a stolen seat from a less-valuable bike makes a perfect complement to a more-valuable stolen bike whose owner took the seat with them in order to discourage theft. See also: Unintended Effects, Evolutionarily Stable Strategy, Arbitrage, Irony. C.f. Hanlon's Razor.