"They like basketball."
Translation:Вони люблять баскетбол.
Re: "Why would you want to leave out the pronoun here?" In many languages, the unmarked (i.e., default) construction lacks the pronoun. Inclusion of a pronoun is marked, perhaps expressing contrast or emphasis, for example. We have learners from many language backgrounds here on Duolingo. For some, it is a learning burden to remember to include pronouns. :)