At the end of the story, the main character is arrested. What are some of the reasons or grounds that he has been arrested for?

I suppose that to be entirely accurate, Leonard Mead is not being arrested and taken to jail.  Mead is going to the "Psychiatric Center for Research on Regressive Tendencies."  Mead is being taken to a psychiatric ward to be studied and "fixed."  
The reason that Mead is going there is because he displays too many "regressive tendencies."  Basically Mead does things that the general population never does any more.  He is a complete outlier.  What's frustrating and great about his regressive tendencies is that they are completely innocent and harmless to the general population (at least by present standards).  Leonard Mead first draws the attention of law enforcement because he's out at night . . . alone.  In the police car's opinion, Mead must be up to no good.  The suspicion surrounding Mead only deepens with each question that the car asks.  Mead is asked about his profession.  Mead states that he is a writer, which the car interprets as being unemployed.  

"Business or profession?" "I guess you'd call me a writer."
"No profession," said the police car, as if talking to itself.

The car then asks why Mead is outside at night.  Mead tells the car that he is walking for the sake of walking, which the car can't fathom. The car even asks if Mead is walking because the air conditioning or TV is broken. Mead admits that he doesn't even have a viewing screen.  No wife either.  It's then that the police car decides that Mead must be some kind of crazy person and is either a danger to himself or the rest of the population.  Mead is promptly told to get in the car for transport to the psychiatric center.