Potential damage to the injector on each use makes it hard to guarantee a refill will work the same way.
Replacing the pen after actually using it seems entirely reasonable - if nothing else, the needle will be contaminated.
I was thinking of inhalers for asthma. The plastic bit is cheap enough, but they could in theory just give you the medicine filled part.
My understanding is the medicine in these epi-pens is fairly inexpensive..
an ampule of 1:1000 epinephrine is about $4. At least that's the price we pay for them on the ambulance. I'm a Paramedic in the US, and we took the Epi-Pens off our units ages ago due to the ridiculous costs. Now we just draw up epi by hand with a typical insulin syringe and deliver it that way.