Simple, Congress has the power to mediate disputes between states of the Union, it's part of the Contract if you will. Seccession would be withdrawing from the Contract, due to previous violations by the other party or parties to it. Once a contract has been violated by one party, the other party has no obligation to uphold it. The North always wanted to have it's cake and eat it too. The Constitution also says that no state can be created out of another without that state's permission. West Virginia was created out of Virginia, without its permission. Virgina was still a state of the Union, if one also asserts that the Constitution forbides secession. You can't have Viginia be a state, and thus Lee and other be traitors, while at the same time it be OK to rip W. Va. away from Va.