Hi David,
There is indeed a high potential for abuse with such a feature. Due to the nature of it I don't think it's possible to prevent malicious users from entering other people's email addresses. Then again, they can -already- do similar damage simply by 'subscribing' anyone they wish, so it might not be that much worse...
This has been suggested before and is in consideration for future versions but I'll move your post over to the suggestions forum so it gets a little more exposure and attention anyhow.
Regards