Has the GPL ever really been tested in a US court?
My non-expert impression has been that open source licenses and "licenses" that aren't ... explicit contractual agreements between parties (i.e. not just click through or some text in a download) do not have a particularly strong backing with law or case law. Whether or not you "can or can't" do something with regards to effectively making up rules that will be enforced by courts with a blob of text in source code or a check boxes on websites is still somewhat up in the air (somebody who knows things feel free to correct my impressions).