A logic puzzle.

The Pillon Robbery

It was pouring with rain that Saturday when Glen Roberts searched his garage for the tool he needed to fix his convertible roof. It wasn’t there, so he decided to look for it later and went back into the house.
The phone rang. He picked it up and it was his friend, Paul.
“Did you hear about the robbery?”
“…..No.”
“The Pillon family across the street was robbed.”
“Really? Who did it?”
“The police don’t know yet. I bet the thief is rejoicing right now, though, getting away with something like that. That family has always been rich. I guess unless they find the criminal, the Pillons will turn out like my family, in financial trouble, I mean. We’ve never been too well off.”
“Well I know, but I can’t relate. Anyway I need to go, maybe I’ll see you later.”
He then heard the doorbell ring and went to answer it. It was the police.
“I’m officer Miller. I’m going to ask you a few questions having to do with the robbery at your neighbor’s house. First of all, do you know about it?”
“Yes. My friend told me.”
“Have you seen anyone suspicious around lately?”
“No, I haven’t seen anyone like that.”
“Where were you today?”
“ I was just driving around, shopping and running errands.”
The police officer paused and looked like he was thinking. He then went to his car and got another police officer and they arrested him.

Why did they arrest him?