Thunder1,
Would invite you to read Law Review Subject Index
It is mostly written by one of the writers of the current USERRA Statute.
Also invite you to read about the 'escalator' clause in USERRA which protects your major service from the time you were hired till furloughed, then again from your recall date. You must compy with your employers instructions regarding recall and you must notify of military duty (two separate events)

Probation comments: Airlines typically have a time in service (one year) for probation.
Two parts: The first applies regardless of status (probationary or non probationary) upon return from military duty.
USERRA prohibits an employer from terminating an employee following reemployment for a period of time. Duty of 31-180 days require “just cause” for termination for a period of six months. Duty greater than 180 days require “just cause” for termination for a period of one year. “Just cause,” however, is not explicitly contained in USERRA or in the USERRA regulations.

Second part: The Department of Labor (DOL), as issued in the "Final Rules" in 2005, provides that if an apprentice or probationary position is bona fide, not simply a time-in-grade requirement, a returning service member should be restored as an "apprentice" at a level that reflects both the experience and training he or she received pre-service.
As airline pilots, we are not typically viewed as apprentices like a union plumber or electrician, etc. We have to meet a skill level beyond an 'apprentice' to be employed in the first place. i.e. meet certification levels (ATP, etc) with proven experience (Hours, size of aircraft, etc). Also, most airlines follow a time in grade requirment that DOL appears to deem as not 'bona fide' probation.
In all practicality, the escalator clause of USERRA requires we be treated exactly like we had never left employment. This is why we can be furloughed while on military duty. The escalator clause works both ways on the seniority list. i.e. you keep your number and come back with same number or relative position. (union or non union).
Additionally, with the USERRA protection of 'just cause' termination regardless of probationary status, USERRA places the burden upon the employer if you are terminated. One would have to have been so far over the top to lose the USERRA 'just cause' protetcion even though not explicitly defined. (i.e lie, cheat, steal, fruad, etc) thjat would get one rightfully terminated even if not on probation.
Summary: Probation is really just a clock, since most if not all just a 'time in grade' requirement. Escalator clause and USERRA protects union and non union airline employees from capricious terminations and protect your seniority.