The practical difference is that the association was in Hsu's professional capacity. That is to say, he didn't decide to join the same bowling league as Molyne

The practical difference is that the association was in Hsu's professional capacity. That is to say, he didn't decide to join the same bowling league as Molyneux as a random guy, he went on Molyneux's show as an academic authority on the science of race and said what Molyneux wanted to hear. I have heard Molyneux's fans say to me that they were drawn to Molyneux's racist ideas "because he just applies science" and pointed to Hsu's interviews (and others) as evidence of this. Hsu enabled that not by association in general, but in this specific form of association which some people have interpreted as endorsement. There are plenty of other professions with ethics rules prohibiting certain associations due to how those associations may be perceived, e.g. large companies have antitrust guidelines that prohibit associations that may cause the perception of anti-competitive behavior.