There should be policies regarding linking to, and sharing copyrighted material in public forums quite independantly of whether they generate any income from advertising.
The reason for this is that it becomes very easy for somebody using a good search engine to find copyrighted material, collect it together and then make money from this collection. While we may agree to disagree about whether scientists should have to pay at all to view publications (i.e. whether all research results should be open access), I think that we would agree that it is wrong for a third party to collect the copyrighted material and make money from it.
I hope that this is helpful to you in understanding the ethical reasons why the Forum may not wish to support this practice.
I also do not think that it is fair to suggest that the Chemistry Forum should not make money from advertsing. They are providing an excellent service and this costs time and money.