Calling it a shooter is a SERIOUS technicality. Such a technicality as to render the term meaningless. The aiming and shooting is not actually a task of trying to shoot something, it is the simplest way to let the player place a Portal. The entirety of the spectrum of skills and action that you take in Portal are completely different from a Shooter. I will argue that World of Warcraft, Zuma, and Missile Command are more Shooter then Portal.
I don't know why people seem to think that Portal needs some odd, niche characteristic to reflect it's genre. Braid isn't a 3rd person Oil-painting-Platformer. Bejeweled isn't a 3rd person Gem based Puzzle game. Tetris isn't a 3rd person vertical downward movement based Puzzle game. Portal is a Puzzle game and a Platform game. The fact that it is 1st person and uses a "Gun" to place the puzzle pieces doesn't change the Genre, it makes up the mechanics of the game.
I don't know why people seem to think that Portal needs some odd, niche characteristic to reflect it's genre. Braid isn't a 3rd person Oil-painting-Platformer. Bejeweled isn't a 3rd person Gem based Puzzle game. Tetris isn't a 3rd person vertical downward movement based Puzzle game. Portal is a Puzzle game and a Platform game. The fact that it is 1st person and uses a "Gun" to place the puzzle pieces doesn't change the Genre, it makes up the mechanics of the game.