In the case of EWR, I'm not sure that there are any trains that stop there which offer checked bags anyhow. The LD trains (as well as the only NER train with checked bags, 66/67) all skip EWR.
Similar story at BWI, where all LD trains skip and 66/67 is the only train with checked bags that stops there.
And at MKA, the only trains that stop there (the Hiawathas) also don't offer checked bags. (The EB does not stop at MKA)
So to argue for checked bags at BWI, EWR and MKA you would have to either make the case that LD trains should stop there, or that current corridor service should offer checked bags.
BUR is a different animal since the corridor in question already offers checked bag service, just not at BUR. I think you could make the case for adding BUR but I would guess Amtrak feels that there is not enough demand there to add a station agent(s).