Yeah I am not against people making money. In this case if just felt distasteful and not properly disclosed.
Most people will probably know that that affiliate link benefits the mod of that community, but many won't. People might just think "Oh wow that's cool, thanks for sending me in the right direction you charitable mod".
The problem is the lack of disclosure and the overall disingenuous feeling, not that someone is making cash.
I disagree, not making it explicit adds a level of abstraction and assumes that everyone needs to know how online affiliate programs work. Users should know, without any question or confusion, who exactly is benefiting and how whenever they use an affiliate link. Otherwise there are perverse incentives.
And its not just me, here is the FTC's stance.
I know FTC is American, but I think its a good guideline.