In the US, a "softball question" is asked because it would be intentionally easy to answer. It's not an intellectual judgement, just a question formulated to be intentionally easy for that particular person to answer.
I'm looking for a word or phrase that can be more widely understood (outside the US) that preserves that same meaning. Preferably, it would fit nicely into the form: "The best political reporters no longer ask [softball] questions."
The best (and non idiomatic) thing that I could think of was "safe"
I don't know a direct synonym. If you have room for a few more words, you could add the words planted and canned to provide a contextual background or explanation for softball. (That is, could write "... no longer ask planted, canned, or safe and easy "softball" questions.")
Planted questions and canned questions frequently are softball questions in the sense of being easily answered, but the three words all have different connotations. If a politician plants a question, questions of ethics may be raised when that's discovered. The term canned is far more frequently applied to answers rather than questions, but web searches show it still being used frequently of questions.
Note, an ngrams comparison of planted/canned/softball + question shows that planted question is used far more frequently than either of the other two phrases; planted question usage has fallen in recent years, and usage of canned question risen.
You might call the questions deferential or ingratiating, under the assumption that the person asking softball questions is doing little more than toadying up to the politician.
A leading question could be a slanted to get the answerer to say something they don't want to. So I would keep it clear and simple and use easy question.
The Merriam-Webster Unabridged defines a leading question as
a question so framed as to guide the person questioned in making his reply.
A softball is thrown at an interviewee in the same (if not strictly identical) spirit.
Non-threatening, perhaps. Favorable, perhaps.
Just to throw another suggesting into the mix: you could be asking a facile question.
The closest non-idomatic phrase I can think of is "favorably biased".
See my comment on Hugo's answer. When we say "softball", we don't just mean "easy", like "How do you spell your name?" The term implies an implied bias in favor of the person being asked. "Softball question" is generally used as the opposite of "hostile question".
Like if a reporter asks a politician, "Why should black people vote for you when you're such a racist?", that would clearly be a hostile, biased question. If he asked, "Why do you think black people aren't supporting you when your policies would create so many new jobs in black communities?" that would be a softball question, biased in the person's favor.
