The ToS says: "You are responsible for any tax implications that apply based on your country of residency and citizenship."

Verdict: Extremely vague! We *know* that no credit card issuer sends out 1099s for credit card rewards, but Citi does for bank bonus awards because it is considered earned interest.

Since United is awarding the miles directly, it's hard to say. Cant think of a scenario in the past in which an airline has directly issued a 1099 for awarded miles as "payment" for some completed task/work.