... justifies behaviours such as ...

Your argument is missing a step, namely the one where you show that those things really are very bad even though this sort of analysis suggests that they do little harm.

[categorical imperative]

It is possible that James doesn't agree with Kant. But if he does, I suggest that he can clearly respond along these lines: "The maxim by which I propose acting is that of acting to maximize expected utility. If everyone does this then perhaps 1000 people will buy drugs from a criminal organization and hence enable it to commit a few more murders -- but they will only do that if the good each of them is able to do by buying those drugs outweighs the (incremental) harm caused by their contribution to the criminal organization, in which case collectively they will do enough good to outweigh those extra murders. I am happy to live in a society that makes such tradeoffs."

But perhaps you are imagining a version of James that would endorse buying the drugs even if that does no good at all, merely on the grounds that the harm done is small. I agree that this (straw?) James couldn't respond along those lines, but I don't see any grounds for thinking the real James takes that view. He hasn't argued that getting a job for an organization that does good would do relatively little good, so there's no value in it; he's argued that getting such a job would do less good than earning a lot of money and giving much of it away.