IMO, # 2 and 3 are terrible reasons.

Re: #2: If bandwidth or storage were a problem, the instance admins could handle it. Preemptively hotlinking to some external site deprives them of the chance to solve the problem in a better way, while also making it more likely that the pic becomes unavailable and leaves an orphaned post.

Re: #3: Another way of saying that is "letting Reddit's corporate censorship usurp the Lemmy mods' and admins' power." Does it still sound like a good thing? Just because fucking Reddit, of all non-person entities, has a problem with it, doesn't mean it's right or that the Lemmy instance hosting it is subject to the same laws!