If you look into the law properly, rather than just a stupid knee jerk reaction, you'll see that it isn't so mysterious after all:
1211.100 Title 14 - Aeronautics and Space
Part 1211 - Extra-terrestrial Exposure
1211.100 - Scope
This part establishes: (a) NASA policy, responsibility and authority to guard the Earth against any harmful contamination or adverse changes in its environment resulting from personnel, spacecraft and other property returning to the Earth after landing on or coming within the atmospheric envelope of a celestial body; and (

security requirements, restrictions and safeguards that are necessary in the interest of national security.
1211.101 - Applicability
The provisions of this part to all NASA manned and unmanned space missions which land or come within the atmospheric envelope of a celestial body and return to the Earth.
1211.102 - Definitions
(a) "NASA" and the "Administrator" mean, respectively the National Aeronautics and Space Administration and the administrator of the National Aeronautics and Space Administration or his authorized representative.
(

"Extra-terrestrially exposed" means the state of condition of any person, property, animal or other form of life or matter whatever, who or which has:
(1) Touched directly or come within the atmospheric envelope or any other celestial body; or
(2) Touched directly or been in close proximity to (or been exposed indirectly to) any person, property, animal or other form of life or matter who or which has been extra-terrestrially exposed by virtue of paragraph (

(1) of this section.
For example, if person or thing "A" touches the surface of the Moon, and on "A's" return to Earth, "B" touches "A" and, subsequently, "C" touches "B", all of these - "A" through "C" inclusive - would be extra-terrestrially exposed ("A" and "B" directly; "C" indirectly). © "Quarantine" means the detention, examination and decontamination of any persons, property, animal or other form of life or matter whatever that is extra-terrestrially exposed, and includes the apprehension or seizure of such person, property, animal or other form of life or matter whatever.
(d) "Quarantine period" means a period of consecutive calendar days as may be established in accordance with 1211.104 (a).
1211.108 Violations
Whoever willfully violates, attempts to violate, or conspires to violate any provision of this part or any regulation or order issued under this part or who enters or departs from the limits of a quarantine station in disregard of the quarantine rules or regulations or without permission of the NASA quarantine officer shall be fined not more that $5,000 or imprisoned not more than 1 year, or both.
Its actually about comtamination from "celestial bodies". In fact, before going on space missions, it wouild be dumb
to have laws about such things. Insurance policies mention "acts of God". This doesn't mean God exists.