Consent must be freely given
“Freely given” consent essentially means you have not cornered the data subject into agreeing to you using their data. For one thing, that means you cannot require consent to data processing as a condition of using the service. They need to be able to say no.
It's just like any contractual clause that would sell yourself in indentured slavery or oblige you to make sexual favors is automatically void, since consent to these things is not something you can sell, you can change your mind about these things at any time no matter what contracts you've signed, because they can't be binding for that.
Just like no company can purchase my kidneys. I can donate them, and hospitals can transplant them, no problem. But they are not for sell.
We know what is being done with human organs when it's on the market for profit. Nothing good. Idem with privacy (especially when your personal data encompass other's: contact details, emails, photos, etc).
The argument is that denying doesn't prevent you from acessing the site.
I recall there's precedent for this being legal, but I can't seem to find it. Search engines have really gone down the drain lately.
I've also seen a Spanish site with this, but as far as I know it has only been accepted by Austrian and German authorities (and challenged recently by some courts).