[Condescending explanation] Among the four freedom, free software gives you freedom 1: the right to private modifications. Meaning, as long as you do not redistribute the software, you don't have to redistribute the source code. And if you do distribute a particular version of that software, only that version of the source code must be accessible. You can still conceal a private fork.
Now in the case of software executed remotely, or software as a service, Eben Moglen himself reckons there is a conflict of rights: because the client is effectively using the code, it should have access to the source code. And because the server do not actually distribute the software, it should have the right not to release anything.
You can't satisfy both rights at the same time. The GPL satisfies one (it is 100% compatible with privacy rights), and the AGPL satisfies the other (and makes itself incompatible with some privacy rights in the process). That's a pity, because before the rise of software as a service, the GPL was the ultimate freedom respecting licence. Now you have to choose between two compromises.
PS: Don't say "property" about software, if you can help it. It is a inaccurate analogy from the physical goods, which fail to convey the truth about knowledge. (Of course, software is pure knowledge.)