Po co Tom by tam poszedł?

Would a better translation be ‘why would Tom have gone there’?

You’re right, the wording of the Polish sentence seems to suggest that it’s about the past (perhaps very recent past).

By the way, another common way to express more or less the same thing would be to say “Po co Tom miałby tam iść?”. It’s mostly used when you want to emphasize your disbelief in another person’s suggestion.