This is a problem if you have proprietary data that you want to use to augment OSM. Some interpretation here: https://wiki.osmfoundation.org/wiki/Licence/Licence_and_Lega...?
tl;dr from that document:
> "...if you improve our data and then distribute it, you need to share your improvements with the general public at no charge. A painless way to do that is to contribute your improvements directly back to OpenStreetMap."
Data sources that people commonly want to blend with mapping data have legal and regulatory restrictions with respect to privacy, data jurisdiction, acceptable use, statutory time limits, contract terms, etc. The OSM license terms are in conflict with legal and regulatory compliance requirements.
The loophole that most companies seem to have landed on is using OSM data to do parallel construction of private data, which technically avoids the Derivative Database problem.