Evil, why even bother faking it! 
I don't know, but it probably looks cooler. Calculator batteries can last for a long time, so solar isn't really necessary. Also, if it's purely solar with no batteries, if the power source cuts out, you can loose your calculations.
Still, solar is a nice addition (when you have battery option as well), and I think recently more of the cheap solar calculators are real than before. I haven't verified this, however.