I don't understand what's "decent" about that. What do you expect the benefit of that information to be, in Consequentialist terms? That people get the opportunity to be dissuaded from clicking on a link they'd have otherwise clicked on--and thereby disuaded from purchasing something they'd have otherwise purchased--because clicking that link makes someone else money with no cost to them? Even though it was a link to something they already saw enough objective value in to click without knowing that?

Would you also like Dropbox to add disclaimers to all their referral links reminding people that the person who sent them the referral is getting extra space as well?

What's not decent is when there is an artificial asymetry of information for no reason. If you tell me it's an affiliate link, at least you are being honest and transparent that the link will benefit you. Whats wrong with being transparent ?