There are (at least) three aspects to this:
1. Your contract with google that you enter by using their services. Obviously we are not here to do googles job and it is up to them to enforce their terms or not, however https://developers.google.com/maps/terms is quite clear on what they in principle allow and not. If you don't want to abide by their terms, don't use google.
2. Any data that you derive from a source without permission creates a potential legal, business and financial liability for OSM and for anybody using the data. While in some legal systems the actual act of copying might be "legal" (but not ethical), the use of such data may have substantial risks outside of the country endangering the whole project.
3. We (the OSM community) are building this dataset, we do not want to use sources against the will of their owners. In general I would find using competitors resources to build a competing product unethical.
The tl;dr version: no it is not ok to use google aerial imagery.
PS: while Mapbox has the same source as bing for lots of its imagery it is always a good idea to check their imagery, sometimes the data will be very different.
