Are Alcohol stoves legal on JMT?

Yes, and no. I would take one if I did the PCT (likely not going to happen) but I know, from efforts other backpackers have made in another forum, that SEKI and Yosemite allow them while Sierra National Forest does not, during a fire ban. Yosemite does not often have a ban on campfires even when all surrounding jurisdictions do. SEKI has elevation bans, but you can use stoves pretty much everywhere (except wood stoves - those aren't allowed unless campfires are, more about the fuel available than the likelihood of forest fires -- if there's one tiny pine at an alpine lake, they def. do not want you to chop it up - yet I see people doing just that at Pear Lake....)

The fire bans do get pretty specific. None in the Sierra get so bad as the ones in the coastal range - in the Ventana Wilderness you sometimes cannot take any stove at all, cannot build fires, cannot smoke, cannot use anything that creates a spark... it does depend on the state of the wilderness.