The New Zealand Cricket Players Association executive manager's comments come in the wake of suggestions by IPL commissioner Lalit Modi that McCullum tried to exploit "a loophole" in the tournament rules.
"McCullum was trying to become a free agent [by threatening not to sign a New Zealand Cricket contract].
"He wanted to play in the entire IPL and there was a loophole there, but that loophole was closed during our meeting on September 2 when we introduced a new amendment for players who don't have a national contract" Modi told the Sydney Morning Herald.
McCullum and fellow IPL-contracted New Zealanders Daniel Vettori, Jacob Oram, Ross Taylor, Jesse Ryder and Kyle Mills delayed signing national contracts over concerns they would lose out financially because of a scheduling clash between the IPL and Australia's tour of New Zealand in March next year.
The IPL laws have now been amended to prevent players from rejecting national contracts in order to play in the lucrative twenty/20 tournament.
Players must obtain a "no objection certificate" (NOC) from their national boards to play in the league.
"We want to ensure people continue to play for their countries. Nobody is going to budge on the NOC issue. It was designed so players do not try to become free agents," Modi said.
"I feel sorry for Brendon McCullum, that he has been highlighted here, because this issue is one that is facing a number of players around the world," Mills said.
"It is all just a bit of a sideshow to the real issue. We all want to see the best players playing international cricket. The only way we can make that happen is not by making up new rules or picking and choosing who players can work for.
"We need to get the structure right and establish a window for the IPL. That will solve this problem.
"The IPL is just reacting to concerns by other stakeholders that players may make decisions to play in the IPL ahead of international cricket.
That is a genuine concern and it is coming to be a bit of a reality.
"But the point is these NOCs can't work. They put the onus on national boards to object and they can't do that.
"That would basically prevent a player from working somewhere else and that would be viewed as a restraint of trade. The boards could therefore be liable for that player's loss of earnings."
It was previously reported McCullum gave up close to $500,000 by signing with New Zealand Cricket and agreeing to a late start in the IPL.
McCullum captained the Kolkata Knight Riders in this year's tournament which was transferred to South Africa.
