Why are "devices utilized exclusively in any transportation vehicle" exempted from FCC compliance testing?
Car and aircraft manufacturers have extensive industry standards and company specifications for EMC/EMI. For cars these standards include CISPR 25 and SAE J1113. Aircraft have RTCA DO-160 and MIL-STD-461. These are in many ways much stricter than FCC Part 15 and are required for safety certifications, such as by the FAA or the European Community Whole Vehicle Type Approval (ECWVTA) process.

The issue is that vehicles are safety critical. As one engineer who works in the field told me, people will drive up to broadcast towers. You cannot have the engine computer sensors misread and cause the engine to explode. As a result, they drive cars into anechoic chambers and use kilowatt transmitters to test them. Likewise airplanes get struck by lightning on a regular basis and must continue to operate normally. 

Typically, if you shield your systems against this energy going in, you will prevent radiation at the same time. On the other hand, consumer devices rarely have legislated immunity requirements, everybody remembers speakers picking up TDMA cell phones.

Car manufacturers will even have standards on how good their radio should work, for customer satisfaction. One of the issues I heard about was that Qi wireless charging pads have the potential to interfere with AM reception, which is why they're often stuck deep in the console.

In sum, the vehicle industry has adequate self-regulation for safety, so the FCC doesn't need to worry.