I think bitcoin can be used as salary when it comes to freelancing in general , bitcoin prices are very volatile, so it 's not a fixed salary for each I think that we need to consider it like that, I doubt a physical business would allow BTC as their payment method anytime soon.
from my opinion earning from freelancing can't be considered as a salary because it was uncertain sometimes you get paid because getting into project or winning contest but the other time you don't,but i call it earning,but unfortunately most of popular freelancing site doesn't use bitcoin or allow bitcoin as their freelancer's receiving method,even there's also bitcoin based freelancing site but don't seems that popular compared to the popular ones