I've been reading further on this, and it's quite interesting.
It seems that NTFS has a more complicated way of storing permissions, using lists of users and what they're allowed to do. So, read-only in NTFS (as I understand) applies per user.
Linux can only approximate NTFS permissions, because it's used in a very different way, having only three groups: owner, group and 'all'.
On top of all that, the driver that we currently use, namely ntfs-3g, doesn't have full support for NTFS permissions. You mount the partition either with full read-write access or with read-only access.
ntfs-3g Ownership and Permissions Support
ntfs-3g File Ownership and Permssions
Communicating With the Other Half: NTFS Support in Linux