IANAL and my expertise in this matter is only as deep as skimming these articles[1][2] but it seems like it would be illegal if Uber were a monopoly and cutting prices to prevent competition. Cutting prices to try to gain market share seems to be a competitive practice which benefits consumers.
1.https://www.ftc.gov/enforcement/anticompetitive-practices 2.https://www.ftc.gov/tips-advice/competition-guidance/guide-a...