OS X can write to NTFS volumes. However, it's a hidden switch, and it's not supported. Use it at your own risk.
OS-level support for a file system is a major technical investment. It's not just about knowing the rules. There are fundamental differences between HFS+ and NTFS, such as characters supported in the name and maximum file size. When writing an OS, not to mention applications, it's very easy to make assumptions in your code that rely on the expected file system.
Reading a file from a different file system is significantly easier than writing a file to a different file system. In only reading a file, there is very little chance of data loss. In writing a file, it is imperative that the act of writing the file does not lose data (either in the file that you are writing, or other files on the disk).
Supporting another file system means that there is a massive expense to the OS development and test teams. If you take a look at this comparison of file systems, you can see many places where these two file systems differ. Without official support for NTFS, the safest assumption is that it hasn't been sufficiently tested and might not support all of the features of NTFS.