SecuROM seems to install 3 things in particular:
1) Hidden registry keys (this is what gets detected as a rootkit)
2) A directory. I don't know if this directory is hidden or not.
3) A windows service that allows non-admin users to access SecuROM functions.
Two things to note:
1) A rootkit is a general description of a set of programs which work to subvert control of an operating system from its legitimate operators. Usually, a rootkit will obscure its installation and attempt to prevent its removal through a subversion of standard system security. Techniques used to accomplish this can include concealing running processes, files or system data from the operating system. [wikipedia.org] SecuROM does not subvert any control of the operating system. Nor does it obscure its own installation or prevent its own removal.
2) If it does hide a directory, it's still as much a security vulnerability as if it were a rootkit, so you're practically arguing semantics if on the topic of vulnerability. Even if it's not subversive like a real rootkit.
There's no excuse for a game to be hiding anything from the OS (and subsequently the user), anyway. It's not a rootkit, but it is retarded.
PS: You can uninstall all of this without a reformat. It's just more complicated than using the uninstaller.