I think that 's not unreasonable. If the non-military retiree wasn't there part of his/her partner's service period, why would/should that person get a 50/50 split of the pension?
People going into late marriages tend to negotiate things like that with pre-nups, i.e., if there's a split, you keep your already accrued pensions and I'll keep mine. In this case, the government is establishing those rules rather than leaving it to a pre-nup.
Looking at it another way, if you weren't married while he served his country, and therefore didn't make whatever sacrifices that involved, why should you be compensated for them with part of his pension.
Don't know the whys and wherefores of the age 60 marriage rule, but I stand by my statement that an unmarried woman between 55 and 60 is old enough to know SS is coming at her fast, and to learn what the rules are.
Anyone who graduated HS can easily parse SS rules, IF they take the time to research it. It's nothing short of astonishing to see how many people don't bother fully investigating something so crucial to their financial future. Instead they lazily take advice from their (decidedly non expert) brothers-in-law, internet blathering by conspiracy theorists, low level beaurocrats at SS field offices who believe all kinds of disastrous, partisan political futures (like SS will be gone in 10 years so you better get it while you can), and other hardly credible sources.
Public libraries and the SS website itself contain a wealth of expert information describing how to navigate specific situations.
I studied and weighed numerous possibilities for MONTHS before coming up with DH & my SS strategy and frequently revisited it to make sure rules and our situation hadn't changed. It's surprising how many friends/relatives have said to me, "How did you know you could do that?" How? I put in the time and effort to learn, that's how.