The movie producers didn't produce a simulation of Britney's voice and attempt to sell access to it.
However you feel about an probably-unapproved celebrity cameo in a movie, it's not the same thing as selling the ability to impersonate that celebrity's voice to anyone willing to pay, in perpetuity.
The biggest difference that I see is that technology has made the simulation cheaper and easier.
The biggest difference here is obviously one of scale. I don't think ScarJo would be threatening to sue you, the individual, if you did a voice impression of her for a talent show or a friends wedding.