Beats me. Someone applying there may be unhirable elsewhere, or have somethign going on that makes looking elsewhere either unlikely or undesirable. Could be someone is after a specific kind of flying; maybe they'd prefer the 737 flying vs. 777 long haul. Maybe the company is simply screening, or actively discouraging, or whatever the policy falvor of the day might be. Perhaps someone simply copied and pasted from another vacancy and didn't think too much of it, such as a non-pilot who didn't understand the implications. I never guess or assume, so whatever the rationale, what's posted, is posted.
I don't have any affiliation with Eastern, but another poster couldn't navigate to the bottom of the page and find the job link, so I provided it for him.