Interesting topic. My personal feeling is that the more motorized- or mechanically-assisted travel we allow in the woods, the less true wilderness we will have. And, wilderness is a fragile and vanishing resource that deserves the utmost protection. Accordingly, motorized/mechanical travel needs to be severely limited.
E-bikes, being relatively quiet and (mostly) pollution-free, have generally the same impact on the resource as horses, and thus, horse access is used as justification for e-bike access. My thought is that, while horses have similar impacts, horses are "grandfathered-in" because they were so widely used for so long before motorized/mechanical back-country travel became possible. In other words, the "horse access train" left the station 500 years ago, and it is unrealistic to say horse travel now justifies recently invented motorized/mechanical travel. Especially considering how inexpensive such modern means are in conjunction with the massive population we now have. Carrying capacity of the wilderness is the true issue, not the relative environmental impacts of the mode of transportation, themselves.
JMHO