ALPA isn't saying it. The NLRB and the NMB are saying it. That "fly now, grieve later" exists in many forms like "obey now, grieve later" and "work now, grieve later." You're organized labor and in the 21st century, just be happy unions have been banned entirely as the Right would like. The protection that you have with the RLA prevents the company from imposing contract terms and sets up an organized way to resolving disputes instead of striking every time someone is improperly junior manned.