The Blood Axes are infamous for Hiring them self as mercs in exchanges for weapons and gear. (Thow alot of times once the fighting if over, the Blood Axes use their new guns agiest the people who hired them.)
It's the reason why the other Ork Clans don't trust them, and accuse them of not being Orky enough.
So because of the Blood Axes, it makes sense for the Orks to be able to be allied by any faction who are not above using Xenos Mercs.
Personally I'm a huge Fan of the Blood Axes, and the allies system has me hopeful they will start becoming more prominent in the Ork fluff instead of being over shadowed by the Goffs/Evil sunz/Bad Moons.