Possible counterargument to the “a license is not a contract” argument:
As things stand, it seems likely that the judge in the case will rule that that the GPL-enforcement lawsuits can be a matter of contract law, not just copyright law, which would be a major change to how GPL enforcement works.
However, there may still be an escape hatch:
ON INSTALLING ALL MODIFIED VERSIONS.
There indeed is not any contractual restriction in the GPL against installing modified versions. There isn’t a restriction against installing whatsoever. From the GPLv2:
Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.
From the GPLv3:
You may make, run and propagate covered works that you do not convey, without conditions so long as your license otherwise remains in force.