I don't think a majority baught it for the price pool, but i am very sure a good amount did. When buying things, it really helps ease us in our purchase, if we get to do it for other reasons than just giving us things. This is why charity or community-supportive buying is so popular. Just look at Dota2 and their Compendium. That item is primarily being promoted as a way to increase the prize pool of TI and that really helps push product.
If you say, that you want to be charitable and then aren't in the end, you are being dishonest and misleading. If you are disclaiming, that you are not being charitable and people yet think you are, then you are doing a poor job of communicating that and you deserve any backlash.
Some people clearly stood with the assumption, that their purchase would help increase the price pool from the very first purchase, and when that was confirmed not to be true, they feel betrayed.