The reason they care is that they're trying to keep the average cost of the awards down. It's the same reason these programs often require a Saturday-night stay and 21 days advanced booking. With these requirements, they will often be able to find tickets for much less than $500 and save themselves some money.

In your situation, it doesn't make a difference, but consider how they'd have to word it otherwise: "21 day advance, Saturday night stay, no open jaw, up to $500, unless the cheapest ticket with these restrictions is more than $500, in which case the restrictions dont' apply." This would be terribly confusing, and make it much more obvious that the purpose of the restrictions is to save them money.