Fallout - Can't join enclave because "they too evil" but can join a legion because "slavery rocks" but doing slavery in the wrong way possible

Well, that's simple;
Enclave were Xenophobic and strive for a lawful wasteland through the ethnic cleansing. They're not even permitting slavery as they consider outsiders are untrustworthy.
Caesar's Legion were band of united tribes strive to bring order to the chaotic wasteland through sheer force and numbers. They uses slavery as a bolster to their crippling logistics.

Lore friendly my ass, a giant aircraft manned by the ncr exiled? boy general oliver must have a huge stroke seeing this crap