Prop Peter Wright, who played 21 tests for Scotland from 1992 to 1996, told the Daily Mail that Scotland shouldn't settle for New Zealand players.
Hardie played for New Zealand at the Under-19 World Championships in 2007 but qualifies for Scotland through his grandmother. He was born in Lumsden, more than 17,000km south of Scotland.
"The SRU can spin it any way they like but here is a player who always made clear he wanted to play for New Zealand," said Wright.
"When it became apparent he wasn't good enough, he threw his hat in with Scotland so he could possibly play in a World Cup and get a club contract."
"Scotland should not settle for second best regardless of how good a player Hardie turns out to be.
"I know the SRU are playing the system and doing nothing wrong, but what kind of message does this send out to young flankers such as Hamish Watson and those coming through the system such as Jamie Ritchie?
"To pick him so late in the day and pitch him straight into the World Cup training squad sickens me to be honest, and if Hardie makes that cut and guys like possibly Hamish Watson or John Barclay miss out, that would be a crying shame.
"It's why I think bringing him into the set-up is demeaning the Scotland jersey."
Hardie had a strong season for the Highlanders but missed the Super Rugby final due to a rib injury.
"Representing Scotland has been on my radar for a long time," the 27-year-old said.
"I'm delighted to get this chance to compete for a place in the squad."
"I'm under no illusions how tough it's going to be to make the final selection but I'm looking forward to getting started and showing how much this opportunity means to me and my family," Hardie added.
Scotland head coach Vern Cotter must reduce his squad to 31 following four warm-up games in August and September.
"We're pleased to welcome John into the group and look forward to him intensifying the competition for places in an already fiercely contested area of our squad, Cotter said.
"Depth is crucial to any World Cup campaign and it will be vital to see the contributions of the entire squad in our four warm-up matches, before making a decision on the final 31."
Scotland play four warm-up matches against Ireland, Italy (twice) and France before their opening World Cup game against the United States in on September 20.
