That's not completely true. What you have to consider is that international law is not really law as much as a set of partially binding agreements and compromises. On the face of it, you're correct. NATO can tell Russia to shove it, and build whatever missile shield they want. But then Russia will tell NATO to shove it when it comes to UN sanctions against countries like Libya, or Iran. And will sell advanced weapon systems to anyone with the cash to pay for them (instead of cancelling on signed contracts to oblige the West). And at the end of the day the Europeans don't want a large, antagonistic, ostracized, and downright hostile Russia next to them. They'd much rather have a friendly, included, and pacified Russia. Which means that they're less likely to support a missile shield in its current form if Russia is dead set against it.
This is why it can pay off big for NATO to make Russia feel included in the missile shield (even if on a purely political level). It can also pay off big for Russia (who wants to be included) to go along with some modified version of the missile shield. Of course the problem is that Russia has hardliners and moderates too, and Russian hardliners are a lot harder to pacify, and Russian moderates need to provide them with evidence that sufficient measures have been taken to insure Russia's safety.
That's why the issue is so complex. At the end of the day everyone is interested in a positive outcome to the missile shield talks. The disagreement is over incremental and marginal differences, over which neither side can lose face, and neither side is willing to trust the other.