Amazon's
standard terms for self-published authors/independent KDP publishers: Amazon keeps either 30% (+charge for traffic) or 65% for copyrighted and 65% for PD books.
It appears IPG had a better deal, which Amazon would not renew. It seems that it was IPG who played hardball.
"They wanted better margin and we were telling them what our terms were. So they took the titles off," according to Suchomel... I would love to sell them the e-titles titles. If they want them they're certainly available."
From these phrases, it seems IPG stuck to their terms and would not budge.