Choosing Beggars

If you buy a product and pay full price for it, that's a fair exchange.

If you get the product free, promote the product and bring more sales to the business, that's a fair exchange too.

What isn't a fair exchange is if you get the product for free by promising to promote it and then your promotion doesn't bring sales to the business. You got a free product, the business lost a product and your promotion was worthless. This specific scenario is what the coupon code thing prevents, and the fact that influencers don't sign up for it means that they're knowingly promising something they won't deliver to try and get free stuff.

Lying for financial gain is called 'fraud', incidentally.

TL,DR; influencers are bullshit and they know it.

It's a business, after all.

Then buy the merch like normal human being. If they wanna get my thing for free to review they better let me curate whatever heck they were saying then call it an ad than review.