ArcGIS Pro is just the name for the current iteration of a desktop suite (after ArcMap). ArcGIS Online lacks many of the „classic“ GIS features, especially raster features.

I think it’s just that the web map viewer is a new product and that feature hasn’t been implemented yet (but I’m not on that team and don’t know their roadmap). And it’s not really a paywall thing since the personal use license gets you both online and desktop apps including Pro.