The MEC at an airline are line holders. They get their line salaries  

... line trip, they get their line guarentee, and ALPA reinburses the loss to the company.   It's called Flight Pay Loss.   They are not earning any salary, they are being compensated for the salary they were gurarenteed.

Nobody on the MEC works 100% for the union, giving up their flying job.   They are full time pilots, who occasionally work for the union and get Flight Pay Loss to keep the same salary they normally earn:

SECTION 5 – PRINCIPLES FOR COMPENSATION POLICY Flight pay loss, in terms of what an officer, representative or member could earn pursuant to the collective bargaining agreement if engaged in line flying, shall continue to be a part of compensation policy under Sections 2 through 4 above. The Board of Directors or Executive Board may further establish policy which recognizes the need for other methods of fair and equitable compensation and days off for officers, representatives and members assigned to part time or full time duty with the Association