This isn't unique to the tech sector, there were companies operating this sort of policy before MS. I'm not asking why, I'm asking whether it is actually economically efficient or whether people just take HR's say-so that it is.
It's kind of the same way in the military, at least for officers.
Once an officer gets around 18 years of active-duty time the law requires that they be permitted to serve 20 years if they wish (to qualify for retirement).
So one of the things the Navy looks at when deciding whether to mobilize an Reserve officer is whether they would possibly hit that 18-year point "by accident" as then they'd have to keep them on the active-duty list for even longer than they thought.
