I'm guessing it's 10%. You don't see 10% of their base not taking because they can't run it in WINE or Linux, do you?
Also worth noting: You bought that from a retailer- who bought it from the studio and publisher; if you didn't buy it direct from them, you are still NOT their customer and even if you did, unless you're running on the platforms they support, you're still largely not viewed as a customer. Trust me. You're trying to map "sensible" on to this. This world, the games industry, is more like the other media industries- it's like the record business. Do you think it's a bright idea to sue your customers? Well the RIAA labels do- because they don't think those people ARE their customers. Same goes for this story we're debating.