I bought some TomTom maps one day ago, however today a screen was displayed "why not download TomTom maps". Clearly unnecessary. At the time of this screen I had Settings > Data Source set to OSM.
I agree, even if I tick 'don't show this message again' it does appear from time to time...
It's a bit annoying that the program can't tell if Tomtom maps have already been bought, we can still have reason to use OSM sometimes (for me I use them when I walk in the forest for example)