Crate Motors-“Not For Use With A/C” Warning

I am looking at re-powering with the Mopar Performance crate motors, either the 320 HP or the 380 HP 360, but each motor description contains a warning stating, "Not For Use With A/C". I want to keep my A/C functional after the re-power, why the conflict with the A/C? ('69 Dart, 273 motor, 904 tranny, 7-1/4 rearend).
Thanks,
Forrest