Well... it can be summarized in three main reasons:
1. Several OSM-based browsers don't display the speed limit (when raising an alert), so seeing the speed in the POI name solves that, and should improve security by not having to fiddle with the browser (while driving).
2. Speed cameras in OSM will evolve to a more complex model than a simple point, using relationships to define the detection zones and the direction of detection. It remains to be seen how the speed limit parameter will be handled in this case.
3. Removing the speed from the POI name in all speed cameras, if necessary, is very simple with a simple regex search/replace applied to the POIs.