It spoiled future content, yes, but I'm not going to discuss semantics when you can simply Google the definition.You can spoil story but not items that u use and/or forget... That spoiled anything? No it's just another sprite in game.
Anyway i think i know who is main character in terraria. It's mentally challenged warrior unable to use chairs. Seriously why i cant use chair? After heavy battle with (Insert boss name here) i'd like to go to my house, take a sit, drink some juice or tea (In real life i don't like/drink alcohol or get any amount of it in any way in anything so i don't do it in game and hate when game don't offer non alcohol drinks. Only thing i can drink in terraria for now is bottled water), eat some sashimi and soup.
As for sitting down, for a player to be able to use a chair every single armour set in the game (including vanity sets) would need sitting down sprites. That's a lot of work for a small cosmetic feature. Furniture sets, on the other hand, are substantially less work (still a lot, but less) for a rather larger cosmetic feature, as they add a lot to the player's building capabilities, mainly through reinforcing a theme.