Uka gave Inari a part of her divine power because she couldn't transform Inari back after she wished to look like Sumizome. However, after Inari gave the power back to Uka, Inari didn't turn back to Sumizome. How could she stay "Inari" without the divine power?
The rules of the god world are not very strict, as we see it proven throughout the series by Amaterasu-sama's careless acts and rash decisions.
Just like the gods didn't lock Inari out of human world for violating the trial rules (not to transform into another human) because that wasn't considered a harmful violation, Inari's "one-time only granted wish" effect of transformation probably wasn't so "one-time".
Or, if you will, it was just the mathematical result of all previous transformations. Because Inari last transformed back into her original form, no more transformations happened after she returned the divine power to Uka-sama. If anything, she would've been transformed back for free, as a gift from Amaterasu-sama for teaching Uka and others a valuable lesson, and to avoid confusion in the human world.
As it turns out, Inari hasn't really lost her powers after giving them back to Uka. Shortly after the point where the anime ends, Inari can see gods again and can use the divine powers again.