If the data is cold/untouched it will auto-tier down to the 7K. Backups hit all data, no matter where it's stored in ssd or 7k. My guess is that the 7K tier was designed to dense and the 7K drives cannot keep up with the demand of the backups but "normal" IO the data goes untouched or rarely touched so it can meet the demand.
You can change your Storage Policies to keep some data in only SSD if you want, but you cannot change the algorithm used for when it wants to tier something down to 7K. If you are only using 20% of SSD, then tell your higher IO/more important stuff to stay up in SSD.