For decades Mirriam-Webster has listed backseat as a noun, and I cringe every time I see it in a novel, which is often. A backseat driver sits in the back seat, not the backseat.
When spoken there is, or should be, a slight difference between back seat and backseat.
Now, finally, at least in the online version, Merriam-Webster lists back seat as a noun with backseat as a less-common variant.
Why is it, or was it, backseat but not frontseat?
And why is it still backyard but not front yard? You have a backyard cookout in the back yard, not the backyard. Again, when spoken, it's different.
Maybe I should just identify as British and use front garden and back garden. With their love for hyphens maybe a back-seat driver sits in the back seat.
It is common for a two-word noun phrase to be turned into one word (or hyphenated) when used attributively (as though it was an adjective). "I sat in the back seat" and "He's a backseat driver" are both common enough (though admittedly lots of people write them various ways regardless of part of speech). I think that figurative use of "backseat" is way more common than any attributive use of "front seat", which is probably why the one-word spelling has caught on for that but not "frontseat".
Some nouns are used attributively so much they gain new meanings that only occur in the attributive -- like "This is a blanket rule" -- I would argue "backseat" is one of those. Written as one word, or with a hyphen, and used as an adjective to mean "micromanaging something one doesn't have control over" (people say all kinds of things: "she was a backseat mother", "he's a backseat DIYer", etc.
"backyard" is the same way -- we use it attributively in phrases like "backyard barbecue" and "backyard baseball". I'm sure people use "frontyard" attributively sometimes too, though I can't recall actually hearing that, but it certainly isn't as common as backyard. That is probably why it's never written as one word.
So, anyway, in conclusion, I disagree with M&W's decision in this case.
