(a) Except as provided by this section, the district may not impose an assessment on:
(1) a governmental entity, including a municipality, county, or other political subdivision; or
(2) an organization exempt from taxation under Section 501(a), Internal Revenue Code of 1986, as an organization described by Section 501(c)(3) of that code.
(b) An entity or organization described by Subsection (a) may contract with a district to pay assessments under terms the district and the entity or organization consider advisable.