I checked before, and if I recall correctly, Time Machine backups are protected at the kernel level -- you cannot modify them, even with sudo. You can delete the whole backup, but you cannot modify part of it.

> but executing things from the backup directory is another.

Time Machine backups are structured in a very non-proprietary way. Each backup is just a folder, protected from modifications, with hard links used to save space. If anything, I'd say good on Apple for supporting a backup format that works exactly like making a copy of a folder.