"1. Because a person is critical of one particular candidate means only that, nothing else."
Ever hear the saying "Silence implies consent?"

If I very vocally criticize only one candidate, it's very likely to be construed by many others as implying I have less to criticize about their opponents.

"2. Whom the person being critical supports is no one's business but [their] own " -
It is, until one opens one's mouth and blabs it. : D

"3. It is entirely possible for people to be critical of the candidate they ultimately end up voting for, when the system is rigged to force "lesser of two evils" voting."
This is true. But research also shows that negative campaigning tends to suppress voter turnout.   YOU may lambast candidate X and then 'hold your nose and vote for her/him," but your putting that down in public may persuade others not to vote for her/him or just not to vote.

"So, I don't know who Tavis Smiley is going to vote for, but it is none of my damn business."   

We're not talking about whom he's going to vote for, we're talking about what he says from his bully pulpit and how it may influence the electorate.

"As a host of a public affairs program on PBS, I would expect nothing less than him examining the candidates critically--that is his job."
If he does it fairly and even-handedly, I'd say, yes, that is, indeed, his job.

Finally, no, I don't know what else he has or hasn't said about the candidates. Nor did I presume I did. : )