However, because you can only access their fork of a GPL project over the internet, there is no requirement for them to provide users the source code of software they're using (which is based on software developed by other people under the social contract of the GPL). Thus users are using software which is technically free software but they have no mechanism by which to excerise any of their freedoms nor even see the source code of a program they use purely because the protocol used to display the interface is HTTP rather than X (yes, it's also not running on the same computer, but from a user's perspective it doesn't matter if it runs on your computer or not -- they are using and depend on the program, even if it happens to be running on a different computer).
Do you agree that this scenario is a problem, and the GPL is not helping to solve it? If so, then you now understand the reason behind the AGPL and how it is clearly in line with the spirit of free software. If not, then I'm not sure you fully buy into the concept of user freedom. User freedom is a holistic thing, it isn't a technical definition that cares about specific implementation details.
I would argue this is a bigger issue than tivoisation, because at least with tivoisation you can at least get the source code (even though you cannot use your freedoms on the hardware you bought). In this SaaS scenario, you can't even see the source code of software you are using.