If ANYTHING is within federal power under the Commerce clause, it's the unhindered authority to establish, by STATUTE, limits on the entitlements that can be granted to corporations by ANY state. It is beyond question that a 'corporation' is engaged in interstate commerce... indeed, in international commerce. The SOLE rationale for the very existence of a corporation is to enable people with assets to engage in some specified project or enterprise without exposing their personal assets to any liabilities stemming from the behavior of that corporation. Thus, at its very core a 'corporation' IS "tort reform." It is a "license to steal" (like a Letter of Marque) ... or do whatever it's chartered to do without exposing its owners to liability for that behavior.
Thus, the statutory liberties granted to those legal fictions can, and should, be steadfastly limited by federal statute. No greater authority need be granted to the Federal government than is already within its power under the Constitution. By raising the bar to that of a Constitutional Amendment, the power of our government is being unnecessarily weakened. SCOTUS, in effect, was merely observing that no such limitation on the activities of corporations were ALREADY established and that EXISTING laws (state and federal) permitted corporate 'speech' without limits inherent in the "license" they have. So, rather than addressing political activities per se, it is only necessary to limit, by federal statute, the legal entitlements that CAN be granted to corporations by any state.
Caveat: IANAL ... but I play one in my imagination.
Afterthought: It's interesting that most folks (sheeple?) think (assume?) that the entitlements granted corporations actually EXCEED the rights of human beings. That is, of course, impossible. The government can only operate with DERIVED authority ... the authority of The People themselves. Thus, the "political speech" we see under the 'protections' of a corporate entity have to be within the rights of people themselves. Thus, there's NOTHING to prevent Sheldon Adelson, for example, from engaging DIRECTLY in the political 'speech' he's funding by proxy to some protection from personal liability inherent in a corporation ... nothing, that is, but the LIABILITY itself.