Is there a law against wearing the shoulder strap part of the seat belt behind back?

Most Likely: YES. In Ontario Canada, the website states:

"A typical seat belt assembly consists of a lap and shoulder belt. The shoulder belt should be worn closely against the body and over the shoulder and across the chest, never under the arm. The lap belt should be firm against the body and low across the hips."

There are limited exceptions, they include medical reasons, a firefighter in a firetruck engaged in their work, even being a person in custody in a police car!

Where you live, laws of course, may be different. I encourage you to check with your local police or government website.

Defeating the purpose of the shoulder belt can be dangerous, it serves to help keep the driver from hitting the steering wheel or the passenger hitting the dashboard during a collision. Airbags are calibrated expecting a passenger IS shoulder belted in. Slamming into an exploding airbag without a shoulder belt to help slow you down first would probably be a (briefly) painful experience!

see: http://www.mto.gov.on.ca/english/safety/seatbelt.shtml