Very simple, the shows pay money and they get a spot on the telecast. You're correct that it used to be only nominated shows that got a spot, but while I'm not sure of the specific year, somewhere around the late 2000s they started letting non-nominated shows pay for spots on the telecast (basically the same as any other advertisements--you put up money and you get a spot). Now with that having been said, there are time restrictions on the telecast, of course, so there may not be a spot for every show that wants to perform. It varies based on what the hosts have planned (although I'd wager CBS would tell the hosts to cut some of their bits before they cut a paid performance) as well as how many nominated shows there are to perform (were there only 2 revivals, have 3 of the nominees closed and don't want to spend the money to perform anymore, etc.). But generally speaking, if a show wants to perform for advertising purposes, the Tonys treat it like that and if they're willing to put up the money and there's a spot available, they give it to them.