I am pretty sure these companies will price in the time to charge and up charge the electricity rates. Considering they wouldn't want to fast charge all the time, they probably price in 2 or 3 hours of time of every rental for charging.
That should translate into savings and/or warm fuzzies for customers in the long-run, though it may take a minute before we get there.
There was no realistic scenario for rental companies to start competing on this axis with ICEs, petrol is an inherently hazardous chemicals royal PITA.