[1] http://www.gnu.org/licenses/200104_seminar.html
Also, being slapped on is not a problem for a contract. We interact with adhesion contracts every day that are slapped on to things. When you accept a valet ticket for parking it has a contract on the back that you are assenting to by using the service. No negotiation occurs and adhesion contracts are valid contracts.
Since "commercially negotiated" is not a term of art, why do we think the GPL is not one?
CoAs require a weaker party, who has no leverage, and it seemingly need to be for a necessity (as part of the "no choice but to agree").
The GPL is an offer, but in no way precludes authors from accepting other terms for use of their work.