Skywest is pretty good.
They like you to pre-assign mil leave before the bid, in which case it builds your schedule around the mil leave. The system gives you 2.5 hours virtual credit for each mil day so it doesn't try to pack a full schedule into the rest of the month, which would reduce your days off. If target line is 85 a typical drill weekend (4-5 days accounting for travel/rest on front/back) would mean PBS would build a 75-72 hour line.
Basically it doesn't force you to drill on your days off, but you can pick up additional trips if you want to get your credit back up...normally there's no shortage of extra flying, although that was not the case in 2008-2009 at the height of the slowdown.
If something military comes up last minute, mil leave will just overwrite whatever was already on your schedule. They will let you drop it at whatever point makes sense for you...you can drop when it passes through your base, or do it at an oustation and make your own way to wherever you need to be.
Other airlines with PBS probably are similar, although I know as a fact not all of them are as accommodating as SKW...mesa even got in the habit of turning off my nonrev/jumpseat privileges on a drill weekend! They did that just to be tools, and it's a violation of federal law. That crap came to a stop when I called them at check-in time for my 4-day...from another city...and told them I would be at work about 4 hours after they turned my CASS back on!
An airline with hard lines will normally just drop all or part of a trip which touches mil leave (unless you bid a line which has days off coincident with drill).