For me, its the idea that they skipped over the wedding part and went right to living as a married couple, making a home together, having children. So, why should I be asked to give them a gift for an event that seems long past even if it wasn't "legal."
It seems to me one of the principle ideas of a wedding is that it is a beginning point, not a midway point. There is an element of "making" something "permanent." If someone was living together for 10 years as man and wife, sharing a home and having children, I would consider them about as "married" as any other married couple--treat them as a social unit, never wonder if they were "going to get married" at some point.
I get that people's personal feelings about marriage change over time and a couple that was unwilling to legally marry early on can change their minds. But, that's not really any of my business. Socially, I am doing my job by accepting them as a married couple because they are choosing to live that way. So, when it comes around, hey, now we're ready to do the whole BWW thing, I wouldn't really be on board with that to the same extent that I would for a couple who is truly starting out and would kind of feel the way the OP does.
I can understand "cuz they wanna." But their "wanna" doesn't turn into the same "wanna" for me as it would for a couple who has not been living together as a married couple for 10 years.
I could celebrate aspects of them deciding to marry but not to the same fullness because they had already skipped over, well, most of it!
And I don't know where the cut off in years would be for me, but I would accept the generally held 7 years which, in many places holds that you are in a common law marriage.
That said, I think a lot of people register at a lot of different stores for a great many things because they get a discount from the store on any of those items that were not purchased later on.
If a couple that has been together for 10 years wants to have a big party and make it legal, that's great. But, if they are expecting a windfall of gifts, that would be entitlement.