How Come I've Seen Right Hand Drive Toyota Supras and Subaru WRX Driving around Detroit?

Everything up there is USA, everything down there, Canada

How come?  Canada is a different country than the US. Since they are a different country, they have different laws. Here in the US, our laws say that vehicles must be over 21 years old to be EPA exempt. Vehicles must be over 25 years old to be NHTSA exempt.

In Canada without getting too crazy about their specific provinces, vehicles over 15 years old are able to be imported. 

As you can see above, Canada is very close to Detroit.  There is a International Treaty called the Vienna Convention on Road Traffic.   This treaty allows people from one country, to drive their vehicle in another country legally. 

One of the main benefits of the convention for motorists is the obligation on signatory countries to recognise the legality of vehicles from other signatory countries. The following requirements must be met when driving outside the country of registration:

• Cars must display their registration number at the front and rear, even if legislation in the jurisdiction of registration does not require a front vehicle registration plate on cars. Motorcycles need display their registration number only at the rear. Registration numbers must be displayed in Latin characters and Arabic numerals. In addition to this, the registration number may optionally be displayed in a different alphabet.
• A distinguishing sign of the country of registration must be displayed on the rear of the vehicle. This sign may either be placed separately from the registration plate or may be incorporated into the vehicle registration plate. When the distinguishing sign is incorporated into the registration plate, it must also appear on the front registration plate of the vehicle. The physical requirements for the separate sign are defined in Annex 3 of the convention, which states that it must comprise black writing on a white oval background and that it must not form part of the vehicle's registration number. In practice, the requirement to display the white oval is mutually waived between some countries, for example between the European Union countries (where the white oval may be substituted by a blue strip on the Vehicle registration plates of Europe),^[2] and between Canada, the United States, and Mexico (where the province, state or district of registration is usually embossed or surface-printed on the vehicle registration plate).

• The vehicle must meet all technical requirements to be legal for road use in the country of registration. Any conflicting technical requirements (e.g., right-hand-drive or left-hand-drive) in the signatory country where the vehicle is being driven do not apply.
• The driver must carry the vehicle's registration certificate, and if the vehicle is not registered in the name of an occupant of the vehicle (e.g., a hire car), proof of the driver's right to be in possession of the vehicle.

China is the most notable example of a non-signatory country. Short-term tourists are not allowed to bring cars into China at all. All foreign registered vehicles in China must display a Chinese vehicle registration plate.

The convention also addresses minimum mechanical and safety equipment needed to be on board and defines an Identification mark (Annex 4) to identify the origin of the vehicle.

There are other types of potential, legal imports, called temporary imports.

• NONRESIDENT OF THE UNITED STATES
• DIPLOMAT OR MEMBER OF AN ORGANIZATION DESIGNATED UNDER THE INTERNATIONAL ORGANIZATIONS IMMUNITIES ACT
• MEMBER OF THE ARMED FORCES OF A FOREIGN COUNTRY
• RESEARCH, INVESTIGATION, DEMONSTRATION, TRAINING, OR RACING
• INTENDED FOR EXPORT

So there are several legal ways for someone to be driving one of these cars in the US. However, if someone is selling one, then check out this article, for some information : Is My Imported Car Legal?