It's an easy anti-spam measure for sure, but there's no reason you (as the user) shouldn't have a checkbox allowing you to remove this limitation. Otherwise it's not a full internet access.
There's a good reason: The demand for it is infinitesimally tiny, and the hassle of doing so (and having to deal with monitoring whether or not you end up being a spam source) is an extra cost.
An argument that it's not "full" internet access will get you nowhere - very few consumer internet subscriptions are "full" internet access in the sense that they are totally unrestricted - most have very severe restrictions on e.g. running servers and many other things.
An argument that it's not "full" internet access will get you nowhere - very few consumer internet subscriptions are "full" internet access in the sense that they are totally unrestricted - most have very severe restrictions on e.g. running servers and many other things.