It seems to me that someone at Waze jumped to conclusions. Having a link to someone else's map, especially via third party tools, in no way creates additional liability on Waze's part, nor does removing such links reduce liability. If Waze is found to have infringing data, they're screwed. If not, they aren't.
Waze can try to put up a legal defense, I suppose, that they did everything possible to discourage users from copying, and this adds to it. But it will be a sideshow to the only real question: is their data infringing or not infringing?
The same thing could have bee accomplished by asking that the button pop up a message before opening the other map window: This link is provided for you convenience in viewing another provider's map information. You acknowledge that you are not permitted to use copyrighted information from another source in editing the Waze map. ACKNOWLEDGE / CANCEL o <- check to not show this message for 30 days.
Most of us make all sorts of non-infringing use of other maps. We submit rpeorts to Google. We view signs on Streetview. Yes, you can get Streetview images on WME, but they'r enot as good -- the Window is smaller, and you can't go back to a previous version of the image if the current version is blurred. There are also some sources whose data is freely available that could be linked but now never will, apparently. (Shhh, in the US, we can even view signs in Bing or Nokia's maps for US edits without infringing. YMMV in other countries.)
I hear the pain of users like (I think) SuperDave, who just won't be bothered to submit changes to GMM, or won't be bothered to verify signage beyond what WME provides. I don't think everyone will personally quit those activities, but they may have to have a more motivated moment to do so, which means the quality of editing will sometimes suffer.
It is what it is, and each editor may do as he or she pleases with this result. No need to get upset about it. Frustrated? Yes. Upset? No.
TL;DR: There isn't really a legal reason to have done this. If you are frustrated by it, and it gets in the way of your editing, feel free to stop using other maps in conjunction with your Waze editing. But don't let it get to you, this is not your real life.
US Champ / Country Manager | State Manager NY, NJ, PA, CT, MA, RI, VT, ME, NH | Northeast ARC | Mentor | Responding to Map Issues