It appears that most of the laws regarding this are local and that they are misdemeanors. Here is what I take it to be a typical one:
Sec. 14-46. - Violations of article.
Unless otherwise specifically provided, a violation of any provision of this article shall constitute a Class 1 misdemeanor. Cross reference— Penalty for Class 1 misdemeanor, § 1-11.
Sec. 14-47. - Required. It shall be unlawful for any person to sell or offer for sale or deliver or have delivered or make otherwise available for use in the city any portable fire extinguisher except those bearing an approved label of the Factory Mutual Laboratories or the Underwriters' Laboratories, Inc.
(Ord. No. 95, § 12; Code 1964, § 18-25)
Sec. 14-48. - Standards for construction, installation, maintenance and use. (a) The standards for construction, installation, maintenance and use of portable fire extinguishers shall be those of and published by the National Fire Protection Association entitled "Standard for Portable Fire Extinguishers," as listed in Appendix A of the 1987 BOCA Basic Fire Prevention Code adopted in Article II of this chapter. It shall be unlawful for any person to install or maintain for use a portable fire extinguisher except in accordance with the above adopted standards.
(b) Copies of the standards referred to in subsection (a) above are obtainable, during regular business hours, at the office of the clerk of council.
(a) It shall be unlawful for any person to remove any seal, tag, label or other essential marking placed on a portable fire extinguisher by the manufacturer or testing laboratories to designate the make, type, use, capacity or contents thereof.
(b) It shall be unlawful for any person to remove, mar or mutilate any tag, seal, stencil or marking placed upon fire protection or extinguishing equipment by the fire chief.
It is a Federal law to tamper with a fire extinguisher no matter what state it is located in. Years ago my brother broke the seal on one. He didn't know that colorless ink was sprayed on his hands when he did it. The Police came while we were still in the store and they used a special light on people's hands and they caught him. It cost my parents $500 in fines(which he never worked off).
The problem with tampering with fire extinguishers is no one will know that it wasn't used. It's really a dumb and illegal thing to do.