The rule that you are referring to is covered under INA 214(c)(5)(A) which states:
In the case of an alien who is provided nonimmigrant status under
section 1101(a)(15)(H)(i)(b) or 1101(a)(15)(H)(ii)(b) of this title
and who is dismissed from employment by the employer before the end of
the period of authorized admission, the employer shall be ...