The reason that students are required to purchase graphing calculators is for tests, not learning; schools believe they have more control over what goes on during a test if all the students are using the same brand of (locked-down) graphing calculator.
There's little reason for a non-student to use one. You'd be paying more for less capabilities, because what you're really paying for is the trust that schools put in its lack of capabilities.
And of course, as every aspiring hacker in high school knows, graphing calculators come with a Turing Complete scripting language that's powerful enough to defeat this stricture for most use cases.
Then again, by the time one is done translating an algorithm into TI speak, one's understanding is as high or higher than a student who spent that time learning to execute the algorithm by hand...
There's little reason for a non-student to use one. You'd be paying more for less capabilities, because what you're really paying for is the trust that schools put in its lack of capabilities.