From the L.A. Times (Doug Smith):

[Seeing that the driver was asleep, and d]educing that the Tesla Model S was [likely] running on Autopilot, the officers called for backup and prepared to employ a stratagem.

After a second unit caught up to block any traffic coming from behind, the original car sped up to get in front of the Tesla, then gradually slowed to a stop.

The cameras and computer algorithms of the vehicle's self-driving system did their job, slowing to avoid ramming the officer's car.

Several miles from the first contact, the Tesla slowed to a full stop ....

Makes sense.