So there are tax treaties that limit how much developing countries can charge in tax, says ActionAid. The implication is that these are unfair. For example, Gambia can't charge withholding taxes on dividends to UK companies.

Is this unfair? The UK doesn't charge UK companies withholding taxes on dividends they make.

I think this is a typical case of someone (in this case ActionAid) deciding how they want the world to be, and then interpreting facts they don't understand in a way that blames someone they want to blame.