This is for anyone else out there wondering. If you disable tone curve settings and then create a snapshot, and then click on the snapshot, the tone curve settings are enabled! The online support guy confirmed it was an issue, that it worked correctly in previous release, said I was the first to report it , and said it will be fixed in the next release. My wild guess: this is something that slipped through the cracks as they were working on other tone curve issues, as I'd think this a function that wouldn't change much through releases.
This bug dates back from at least LR 3. See this bug report in the official Adobe feedback forum: Lightroom: PV2012 snapshots not saving ToneCurveEnable setting | Photoshop Family Customer Community . I just tested LR CC 2015.5, and the bug was present in that version too. So the support person was mistaken (which happens too often, unfortunately).
Please add your me-too vote and details of your issue to the bug report. Feedback in that forum often influences Adobe, and who knows, maybe a new product manager will notice the bug after your feedback floats it to the top. Adobe does seem to be paying more attention since last fall.
An aside for anyone having this problem: it only exists on snapshot creation. If you create a snapshot, and only afterwards disable tonecurve, then right-click update the snapshot - then it will work properly.