Casey, a LIV member who is ranked outside the top 100, has never won a major and meets none of the selection standards, will tee it up at the year's second major at Oak Hill Country Club after being extended a special exemption.
Asked how the Briton, 142 in the world rankings, was awarded a coveted spot, the PGA of America's chief championships officer Kerry Haigh explained that it was because they could.
The case for Casey is built around his play at recent PGA Championships, where he finished in a tie for second in 2020 and fourth in 2021 before missing last year due to injury.
"The process for inviting players who are playing well enough and good enough we will consider no matter what tours they're playing on," said Haigh. "Some LIV players were invited this year, some Japanese Tour, Australian Tour.
"We look at all tours, all rankings, and all players' abilities.
"Paul Casey, great player. He's certainly had some injuries. Played on the Ryder Cup and has played well in PGA Championships.
"We review all those criteria for a number of players.
"We also have the opportunity to invite other players who are not in those exempt categories."
While players who jumped to LIV Golf have been banned from playing on the PGA and DP (formerly European) Tours the four majors have left the doors open to members of the Saudi-bankrolled venture as long as they meet qualifying criteria.
The official world golf rankings (OWGR), past champions and major winners make up much of the majors fields but each has some wiggle room for exceptions.
"What we're about this week is having it be the greatest field in golf," said PGA of America CEO Seth Waugh. "We said that's what we wanted to do, and that's what we've done.
"Everybody who's here this week is our invited guest."
The biggest challenge facing LIV Golf is to be recognised by OWGR so players have the ability to earn ranking points.
LIV Golf's application is currently under review by OWGR with a decision expected in the coming months.
Without world ranking points LIV members will continue to drop down the list, putting their participation in future majors at risk.
"There has been healthy back and forth," said Waugh, a member of the OWGR’s board of directors. "It has not been acrimonious.
"There's been collegial back and forth of them making an application as other tours have done.
"We've responded; they've responded.
"This is not an us versus them.
"I think the OWGR, if you take a step back, the whole point is to create a level playing field, a yardstick by which to measure the game."
