Because remember the infamous Melissa macro virus of 1999? You never know when it's going to come roaring back. By restricting to merely viewing the document, Word is protecting you from any viruses that may be embedded in the file.
This is, believe it or not, a feature, even if it comes across an annoyance.
If you're routinely on the receiving end of Word documents and have a current version of Microsoft Office, you may have noticed something: Whenever you open one of these file attachments or downloads, Word opens in something called Protected View.
