That is an interesting question without a good answer. As I understand it (I am far from being an expert), there are sometimes specific exemptions to allow that sharing. However, generally speaking, I think it is a gray area. The journals would surely be shooting themselves in the foot, though, if they tried to sue their contributors. Academics would bring down hell on any journal that tried that. Moreover, it is not clear to me that they would win as it seems to be a customary practice even if it is not an explicitly allowed one. Finally, sharing a single paper might get research or academic exemptions as you aren't copying the whole journal issue. I doubt any publisher wants to go down that road. They are probably better off with the law remaining vague.