I understand the reasoning for surrendering an almost assuredly lost game, I had one last night where people refused to surrender even though one person screwed the entire game from champion select, and it went the way of Fiora & Master Yi both being majorly fed good luck stopping that split push. Sometimes I just want to be out of a bad game to attempt to get into a fresh one that has a chance at not being a nightmare.
That all being said, you're absolutely right, the first tower down doesn't declare the winning team, now if that first tower went down because the laner got bored of farming your laner for gold and xp and is now so fed they can barely walk, that's another story.
Another problem is that in unranked games, typically teams take turns throwing the lead, or if one team is predominantly in the lead they don't know how to finish the game. That game can go on for sometimes 30 minutes longer than it should have, and could have been surrendered and looking for a new game.
Right now you're low level and you'll eventually be able to see when a game is very unlikely to turn around, but at the moment, just about everyone that you're playing with, unless they're smurfs, will just want to immediately surrender when they lose a tower.