@jsimps thanks for your question! You are correct that indoor mapping is not the objective. Some of the complications include the fact that it's nearly impossible to get a GPS fix indoors and therefore geotagging the photos takes a lot of manual work, and also that Mapillary is not built for handling different levels that buildings have.
With that said, it has been used for indoor mapping in some cases - with one prominent expert on this being our French ambassador @ZIMMY. For instance, here he has captured the Gare de Lyon in Paris: https://www.mapillary.com/app/?focus=photo&pKey=roqA3fapuQApEH_W0Kj4Qw
But as @tryl said, with regard to privacy it is best to get on the safe side and get explicit permission, if you feel like for your personal case you're able to solve the other indoors-issues (i.e. are up for manual geotagging and do not have floors).