I've heard from Harav Mansour Shelit"a as well as other Hachamim that Yitro was rewarded by getting a Parasha named by him. With that in mind, why did Balak get a Parasha named after him?
יתר, על שם שיתר פרשה אחת בתורה (להלן פסוק כא) ואתה תחזה.
[He was called] Jether (יֶתֶר) because he [caused] a section to be added (יִתֵּר) to the Torah [namely]: “But you shall choose” (below verse 21).
This was drawn from the Sifrei on Behaalotecha.
In the popular retelling of this midrash, this was that he Yisro a parsha named after him. It is worthy to point out the difference between parasha and sidra in the language of Chazal. A parasha is the Biblical text from one gap to the next (petuchot and setumot), or in another context, the Torah portion of Eretz Yisrael, where they finished once every three years. Meanwhile, the term sidra refers to the Torah portion they read in Bavel, and which we read, in which we finish every year.
I don't think there is any need to justify the popular retelling of this, in which it is naming rather than adding, and it is the modern redefinition of parasha rather than Chazal's use of the term.
Besides that Chazal called it parashat Bilaam, as Double AA noted, the naming of parshiyot generally come from their opening words, and we need not find special merit to Balak.