From Takepart.com:

In a lawsuit filed Oct. 15, Dallas Safari Club, Houston Safari Club, Conservation Force, Knowlton, and others argue that Delta’s ban on big game trophy transport is unlawful, “robbing the species of the enhancement tourist hunting provides,” the suit claims.

Essentially Knowlton and the pro-hunting groups are arguing two points: Delta can’t discriminate against what its passengers can transport if it’s been deemed “legal cargo” by federal authorities, and the new ban is hindering conservation efforts raised through trophy hunting permit fees like Knowlton’s.