As with any other link on SE, we should encourage people to summarise the relevant content of the paper rather than just linking to it. This might include an excerpt if appropriate.
I think that being strict re Fair Use rules is probably best avoided, since that is purely a US concept and many papers are published in other jurisdictions. Rather than trying to juggle many legal systems, I suggest aiming for "reasonableness". Note that IANAL, however, and SE may have a "corporate" view on this.
A related question is what to do about people linking non-paywalled versions of papers, since these can be legitimate or not depending on many factors. My view is that these should be given the benefit of the doubt, unless somebody alleges that there is an infringement.
This discussion on meta.academia.SE is probably relevant: https://academia.meta.stackexchange.com/questions/752/link-to-free-pirated-papers-or-official-versions-behind-paywalls