If Maglight had written do not use alkaline rechargeable or alkaline reuseable batteries than I think their statement would have made a lot more sense.
The rechargeable alkalines also called reuseable alkalines are garbage. But I think the warning is that under continued use with a real filament lamp drawing over an amp, that they will get to the point where they will leak and eat up the inside of the flashlight faster than what you would think regular alkalines would do and they are probably getting tired of giving away free flashlights.
I don't know if any one still bothers to make the so called rechargeable alkalines, I had some for a while with the special charge they took and they were total Cr@pola.
I use Eneloops in my Minimaglite LED and have never had any probs what so ever, certainly performs far better than any Alkaline thats for sure. I would of thought Mag Instruments would of had a the brains to work this out, but obviously not.