Um, I would challenge the idea PEMF "can't hurt" (as well as the idea that things only hurt or help). We have no flippin' idea what trauma to nerves PEMF may cause.
What we do know is that accumulated trauma from toxins, injuries, possibly certain electrical exposure types, plays a role in who gets ALS. We know that the nerves are a battery and when muscles are overused in ALS, progression gets faster for some period of time.
Nerves control muscles via electrical impulses (think about an EMG that stimulates muscles to move), and an avowed goal of PEMF is to "stimulate neurons." In ALS, stimulating motor neurons artificially could very well cause them to wear out faster. No way I'd do PEMF (and like some others here, as a non-PALS, I have had TENS for pain so I'm not anti-electricity).
And yes, I've seen the papers about the possibility that PEMF helped cell lines mimicking other CNS disorders in vitro to self-repair. Those are not cells in actual people, let alone PALS.