[Oh, because this is exactly the sort of encounter he would have today. Jack has been out exploring for awhile and Ianto is out looking for him. He's not worried about his safety so much as the sort of trouble this version of him could cause and the last thing he wants to deal with right now is Spike.
He almost turns around and leaves when it occurs to him that this is probably his best chance of finding Jack. The first thing he would be likely to look for is John, after all. So he arranges his features into only mild irritation rather than outright dislike and head over to the vampire.]