The purpose of this restriction is to limit the number of people going thru the checkpoints so as to reduce lines, and allow the security screeners to spend more time on the people who have to go through.
Of course, getting through when you're not planning to fly is very easy. You can get an itinerary and print it out. You can even save the HTML and make your own without actually booking anything. Or, you can go and get a refundable Y fare for travel that day and use it to get in, and then refund it on your way out. That's not the point.
The point is that it does have the effect of reducing the number of people going through the security checkpoints, which is all it is designed to do.
The people who are really penalized are the ones who frequently change travel plans while in the car on the way to the airport. Yes, it's easy to get a confirmation or receipt, unless you often come out of a 1pm meeting planning to drive to your hotel for the night, when you check your voicemail and find out that you've been ticketed on a 3:30 flight to Omaha for a dinner meeting.
I'm sure I'm not the only one who has a stack of open Y tickets on my regular routes. I can no longer use them without stopping at the ticket counter.
But overall, even though it does inconvenience people, this seems among the least odius of the new regulations.