That's a fair question, actually. There are a lot of things about this game that people take only at face value, and never consider anything beyond that. I mean, there's a monster-thread about civ-physics or something like that.
In this game, gold (as in the currency of the game) doesn't represent just money - you can't say 1 gold = $1 million or something like that - but rather represents an amount of labor that was used to produce things. Some of these things might be whole units, or they might be spare parts, or pre-fab pieces, or whatever. A city producing gold can help a city producing a Wonder, if you're using US, by helping you to buy the last of the production.
It's not that you literally bought it, it's that the gold represents stockpiled resources and parts and labor. There's kind of a handwave that assumes your people knew which parts would be needed slightly before you did, AND that nothing goes to waste, but that's mostly for keeping people from crying about wasted hammers and such. Previous editions of Civ did a lousy job of handling overflow.
Okay, so with all that out of the way, what about the hammer-to-gold overflow from building GMs in your Ironworks/HE/MA/Forge/Factory/Shale Plant/PS/SP city that gives +395% military unit production on 80 base hammers, plus a chop?
In this case, you're just going to have to accept the 1-thing-per-turn-per-city limit as the implementation of fairness that it is and pretend that all that gold is spare parts for whatever you need later. If all those hammers disappeared, I think you'd hear a bunch of crying on the forums about it.
I realize this isn't the strongest possible argument, but I never promised I had such for you.
