YSK: While you're on Lemmy/Kbin/Fediverse, you're not "the product" but you're also not "the customer".
Why YSK: Getting along in a new social environment is easier if you understand the role you've been invited into.
It has been said that "if you're not paying for the service, you're not the customer, you're the product."
It has also been said that "the customer is always right".
Right here and now, you're neither the customer nor the product.
You're a person interacting with a website, alongside a lot of other people.
You're using a service that you aren't being charged for; but that service isn't part of a scheme to profit off of your creativity or interests, either. Rather, you're participating in a social activity, hosted by a group of awesome people.
You've probably interacted with other nonprofit Internet services in the past. Wikipedia is a standard example: it's one of the most popular websites in the world, but it's not operated for profit: the servers are paid-for by a US nonprofit corporation that takes donations, and almost all of the actual work is volunteer. You might have noticed that Wikipedia consistently puts out high-quality information about all sorts of things. It has community drama and disputes, but those problems don't imperil the service itself.
The folks who run public Lemmy instances have invited us to use their stuff. They're not business people trying to make a profit off of your activity, but they're also not business people trying to sell you a thing. This is, so far, a volunteer effort: lots of people pulling together to make this thing happen.
Treat them well. Treat the service well. Do awesome things.
This has been commonly spread around Reddit for a while, and is completely made up.
The full original quote was "The customer is always right." This was pushed by some retailers as a way of setting the standard of how to treat customers.
Like most oversimplified phrases, it can't be used as a blanket policy, because customers take advantage of it. "in matters of taste" is a nice way to try to correct the phrase in response, but it was never the "original" and it does no favors to revise history to cover up the blunder.