Its also rational if it allows them to move away from custom designs entirely, and save a truckload of money, more than they lose to the nebulous spectre of "piracy".
People usually cite narrow cases where an AAA game succeeds in spite of lacking DRM as if piracy helped, or games that didn't spend anything on marketing and get big through marketing, which is an exceedingly rare lottery to win.
No one ever cites games like Crysis 3 which had been pirated more times than it ever sold days before release
Piracy will obviously cause some non-zero number of people not to buy games because it turns a games cost from a price, to a suggested donation.
"I'll pay if I like it" isn't the profit model a publisher who puts out a multimillion dollar game wants to support.
Not to mention, paying for something can be a great motivator. It's a lot easier to dismiss a a game saying "I wouldn't have wanted to pay for this" at the slightest flaw when you haven't paid for it.
But today, you can do it with a few clicks, so if someone really wants to play a game because it looks cool/interesting/whatever, I'd say a lot of people will just pay for it if they can't find the crack.
The major boost in sales was improving the sales process (Steam and similar stores) and dropping prices, not attacking legitimate users with DRM rootkits. People keep repeating this DRM suppor mantra when up until now there STILL hasn't been any good proof that it does any good. If anything, it only damaged sales due to people not buying games that randomly stop working when internet fails.