Well, there's a few things about this game being pirated. One is that most people who pirate it wouldn't buy it (we'll say 80% of that 90% wouldn't have bought it). Also, a small percentage of those that pirate it will buy another game from the same developers if they liked it (give another 2% there).
This, of course, means that by very rough estimates the developers are only seeing 5/9 the profit they should thanks to piracy.
However, to their credit, adding DRM would've reduced piracy but also legitimate sales, probably by the same portion, which means they ultimately came out ahead by choosing not to implement it, never mind the high cost of ACTUALLY implementing it.
Don't fool yourself into thinking piracy by a single individual doesn't hurt. I pirated every PC game I could find for several years, and had I bought them instead (within my budget, so not buying as many as I pirated by a long shot), I'd probably have bought 10 games. There are at least a few million "old me"s out there, and that amounts to a few tens of millions of games that didn't sell. Piracy hurts.
But then there's the "gotta support the [devs/artists/etc]!" clowns. I won't get into that, but suffice it to say that yes, buying at full retail sends more money to the developers, but if you want to support them instead of the retail stores, buy at a good price or used and send money the developers' way in some other manner. But also remember that supporting Wal-Mart by buying a game from them = Wal-Mart seeing profit from video games = Wal-Mart buying more video games from their distributor = distributor making more money = distributor buying more games from a given publisher = publisher making more money = publisher signing higher-priced contractors with developers. Sure, you're not supporting them as directly, but it's all the GAMES INDUSTRY and it's still a business.
...Wow, where did I get off talking about piracy for a page?
...World of Goo is a good game. Buy it at $5.