I could be that the DDNS providers don't want a large number of people signing up for their serivce for no real reason. I mean, chances are a lot of Tiger users would sign up for it simply because it is there, not because they run servers. Okay, so after typing that out it doesn't seem that plausible anymore... Ahh, well.
Personally I love the idea of having it in the OS, I use dyndns.org all the time (note the website link in my sig) and I find it to be something really useful.