Yea, it's fine to have straight pipes on a bike, but not on a car. Just like how a truck driver can't use his jake brake in some neighborhoods to help him slow down, but to have straight pipes on a bike that puts out twice as much noise as a jake brake, there's no problem with that.
As far as comparing riding in the back of a pickup with riding a motorcycle, if you slam on the brakes on a bike, you don't go flying through the back window and then through the windshield. If you have real seats mounted back there with seatbelts, that's a different story. When a friend of mine was bringing his Pontiac Fierro (one of the shittiest cars ever built) down to the junkyard, I grabbed his seats. I welded angle irons onto the seat brackets and made a frame that fit the bed bolts in the back of my Ranger. I had the two seats back there against the cab facing backwards with seat belts on them, and it passed inspection two years in a row. I got pulled over a few times by cops who just wanted to make sure the seats were actually secured.