Here is the law in its entirety

§ 46.2-1049. Exhaust system in good working order.

No person shall drive and no owner of a motor vehicle shall permit or allow the operation of any vehicle on a highway unless it is equipped with an exhaust system of a type installed as standard factory equipment, or comparable to that designed for use on the particular vehicle as standard factory equipment, in good working order and in constant operation to prevent excessive or unusual noise. An exhaust system shall not be deemed to prevent excessive or unusual noise if it permits the escape of noise in excess of that permitted by the standard factory equipment exhaust system of private passenger motor vehicles or trucks of standard make.

The term "exhaust system," as used in this section, means all the parts of a motor vehicle through which the exhaust passes after leaving the engine block.

Chambered pipes are not an effective muffling device to prevent excessive or unusual noise.
(Code 1950, § 46-305; 1952, c. 455; 1956, c. 651; 1958, c. 541, § 46.1-301; 1960, c. 120; 1970, c. 266; 1972, c. 66; 1989, c. 727.)

The only part that you and most other people concentrate on is the first part:

"No person shall drive and no owner of a motor vehicle shall permit or allow the operation of any vehicle on a highway unless it is equipped with an exhaust system of a type installed as standard factory equipment,"

However the second part of the law reads:

"or comparable to that designed for use on the particular vehicle as standard factory equipment,"

The key word in the above line is comparable. Comparable is defined as:

com·pa·ra·ble ( P ) Pronunciation Key (kmpr--bl)
adj.

1. Admitting of comparison with another or others: “The satellite revolution is comparable to Gutenberg's invention of movable type” (Irvin Molotsky).
2. Similar or equivalent: pianists of comparable ability.

Using the a dictionary definition of the word comparable, the law only requires similarity to the original equipment system installed on your car. In most cases this means a pipe after the catalytic converter followed by a muffler, or in some case a resonator and then a muffler.

Think of it this way. Is a Nissan Titan comparable to a Chevy Silverado? They are made in different places, out of different materials, and they share no parts, but no one would argue that they are not comparable simply because they are both trucks.

I haven't forgotten about the word equivalent either. Look it up and you will see that it follows suit in definition requiring only similarity between two items.

The law does not limit you to buying and installing only factory parts. It leaves plenty of room to install many aftermarket systems, or to go to a muffler shop to have a new exhaust installed if your original rusts off.

Think about the muffler shop replacement for a moment too. Is it standard factory equipment if a muffler shop bends your pipes and uses their own brand of muffler? No. Why? You didn't buy it from your manufacturer which is the only way to get standard factory equipment. So using the arguement that you can only have stock exhuast is completely wrong, unless you think that everyone pulling out of Meineke after having their exhaust replaced deserves a ticket too.