Small jet flying in the Delta contract is "Permitted." The word "permitted" means:

1. To allow the doing of (something); consent to:
2. To grant consent or leave to (someone); authorize: permitted him to explain.
3. To afford opportunity or possibility for: weather that permits sailing.
v.intr.
To afford opportunity; allow: if circumstances permit.
n. (pûrmt, pr-mt)
1. Permission, especially in written form.
2. A document or certificate giving permission to do something; a license or warrant:

Permitted does not mean relinquishing all rights to, giving away, or allowing someone else to trade. If I permit you to sublease my home, that does not mean that you can sell it, or claim it is yours by an act of adverse posession.

Should one MEC be able to unilaterally change another pilot group's contract?

Not that any sypathetic readers are expected on this side of APC, but if we let one pilot group walk in and start negotiating with another pilot group's management we are going to have a complete mess of contradictory contracts. Worse, when we get to our amenable date it will be an open auction if we let management decide who they are going to make a deal with first.