The same is not true of sales comp. Salespeople earn commissions on money they are bringing into the company. It's much easier to measure and while it's not easy to perfectly align incentives (which is why sales teams have spiffs and regions), it's at least possible.
Remember also that the alternative to "no sales commissions" probably isn't "lower paid account managers"; it's "account managers paid nearly as high, but on a salary basis". The best sales account managers can virtually print money; the median sales account manager can't sell bottled water in the Sahara Desert. Most companies that do direct sales need to attract talented sales teams.
But even with a perfect sales team, most products have sales cycles stretching weeks-to-many-months. Which means it can take a quarter or two to see how a sales account manager is going to work out. If you're paying them fixed comp, that's an awfully expensive experiment to run.
I have no idea how well this works with "inside" sales teams ("dialing for dollars" operations); maybe fixed comp makes more sense there.