Listen, if I were in their shoes, I'd be upset about the situation, too. It sucks that we don't have enough manning to let these guys out after four years and take it easy during the reserve time. But unfortunately, no one can instantaneously expand the Army and Marine Corps. I also have a feeling that recruiters told them something like "oh yea, people are rarely called back for the 4 years reserve so you'll be okay" and then bam they're hit with a long ass deployment.
However, there are multiple clauses in the enlistment contract about the possibility of having to do long deployments (24 mo, IIRC), and if you haven't done one by the time you're due to get out, they can give you one up to the day before you are due for discharge and it extends your time. Basically, all this stuff is in writing when they signed the deal, so they have to at least expect the possibility of it actually happening one day.