I think the line can be unambiguously drawn at whether or not the celebrity is receiving money as payment to post something. In the example you've described, things posted before the band had a deal with Ernie Ball don't need the endorsement line, while things posted as part of the contract do.
The point here is to give people true information to make an educated buying decision. People can look back and see that you used Ernie Ball strings when it wasn't part of a contract. And now that you have a contract with Ernie Ball, they can make an educated decision whether they believe that you're still using the product because you like it or whether you are just using it now because you are paid to.
This creates an incentive for companies to create a good product that people will promote on their own, rather than creating a bad product and paying people to promote it.
there is still some ambiguity over whether all these kinds of contracts need to be revealed to the public. If nike pays a basketball player to only wear nike shoes but never asks him to speak or post anything about nike does he have to reveal that contract or what if he does not receive any money nike just starts sending free shoes to basketball players with no strings attached should they have to reveal they are only wearing the shoes because they got them for free? what about if a youtuber just receives free stuff to review? do they have to reveal that they received the items for free?
To answer that with a question, what taxes will each party declare? Is it a gift with no string attached, or a trade of goods for services? When it comes to taxes, this kind of distinction is often very important.
The Swedish tax authority has several documents on how they judge such cases, and in general, a company doing this is unlikely to be seen as a "no strings attached" gift. If it comes from an advertisement budget, someone somewhere is expecting a return on investment.
Just wanted to note that you addressed this first and I hope that I added a little something to your point with my response. You touched on a very relevant point.
So I think you might benefit from a bit of commercial business dealings that offer context: The NFL has an exclusive contract with Nike. A player, Robert Griffin III, signed an independent endorsement with Adidas. That means to do his job, RGIII could only wear Nike branded shoes on the field. He protested by taping over the Nike logo when he was out on the practice field, as a point of principle.
The league - his employer - reminded him of the conditions of his employment. There was nothing preventing him from wearing Adidas from HEAD TO TOE as long as it wasn't in a direct situation sponsored by his employer. Thus, he actually had TWO sponsorships in some terms.
He wanted to wear his Adidas on the field, but was not allowed, even though it was completely above the board. So, where do you draw the line? Can your employer tell you what brand of clothing to wear? If he got the shoes for free, should RGIII be allowed to undermine the other contract?
My perspective is that the line of reasoning you present is inherently flawed. Are you not aware that Nike essentially got its foothold (pun intended) by giving away shoes to college basketball programs and people who probably couldn't afford to buy them? It was covered in an ESPN 30 for 30 so there's the citation.
From your context it sounds like you believe - maybe it's my interpretation - that things given to celebrities for free are "no strings attached" and I think that's been proven wrong time and again. Contracts, handshake deals, they don't work that way. There's either an explicit (preferred) or implied (bad) debt to be paid. It takes time to learn how these systems work, and woe to those who might minimize their practical implementations.
Oh, and a Youtuber who receives free stuff better disclose it, because if they're in the US, then they have the IRS to deal with. The IRS doesn't screw around with compensation. Dollars or donuts, keep the receipts[1]!
[1] Excluding Mitch Hedberg, who accurately pointed out a donut in regular guy life does not need a receipt.
The point here is to give people true information to make an educated buying decision. People can look back and see that you used Ernie Ball strings when it wasn't part of a contract. And now that you have a contract with Ernie Ball, they can make an educated decision whether they believe that you're still using the product because you like it or whether you are just using it now because you are paid to.
This creates an incentive for companies to create a good product that people will promote on their own, rather than creating a bad product and paying people to promote it.