You seem to be missing a rather large point
Yes, frequently. The condition of man, etc. etc...
the bartender is unlikely to use his position to promote his views in the way that Card can.
Indeed, it would be hard for a bartender, qua bartender, to do so. Bartenders have a relatively small audience and people mostly don't go to a bar with political questions in mind that don't involve artificial turf. If a bartender were an ardent follower of Fred Phelps and using his time at the bar preaching against homosexuals, then, as others have noted, he would be doing his job as bartender. If this is the case, then by no means ought one to get paid for work he doesn't do. Likewise, our evangelical would be justified in withhold payment if the barista informed him he didn't serve people who walk around with WWJD bracelets on. The barista, qua barista, is failing at his job.
When you speak of the bartender 'using his position' and the artist 'using his position', however, two different things seem to be going on. When the bartender uses his position as bartender to oppose homosexuality, he hangs a flag in the bar. When an artist uses his position as an artist to oppose homosexuality, he does not do so by acting as a board member of an anti-SSM advocacy group. Rather the artist, qua artist, creates anti-homosexual art. Anyone pro-SSM would rightly regard this art in the same way as a bar with such a flag hanging in it. But GP does not complain that the art is anti-homosexual. Indeed, as far as I'm familiar with OSC's art, he treats homosexuals as sympathetically as any other characters and, given his politics, his inclusion of homosexuals is a credit to his tolerance that his detractors here have not recognized. I think you're quite right, therefore, that GP has deemed the person of OSC unfit for payment.
The analogy, therefore, is not between OSC and a bartender who hangs dehumanizing flags in his bar, but between OSC and a bartender who treats gay patrons as decently as straight patrons, but went to Liberty University and gives money to Focus on the Family. Should such a bartender be rejected? I think we should hesitate before we deem a person unfit to receive money for services they provide. Many have made analogies involving neo-Nazis or KKK members or the like, but I think it is telling that they should go so far. OSC, qua citizen and fellow countryman, opposes SSM. This puts him in league with James Dobson, to be sure, but also with most Americans just a few years ago--including our current President.* Popular opinion on SSM has changed quickly, very quickly if how fast these things have developed in the past is any indication. I would suggest that regarding people as unfit to be paid (for the things they do which we otherwise appreciate) just because they're views haven't changed as fast as everyone else's is not without danger of its own bigotry and intolerance. SSM will win the day (polling and demographics SSM to be the de facto victor; but I think we'll all be better off if the side which loses isn't labeled heretic or thoughtcriminal or unfit. If we can do this, maybe we can get around to enjoying art and other aspects of life without everything being politicized.
*Of course, I do not mean to imply that the moral and ethical question is one of popularity. Only to indicate that OSC's views are not so far outside the the mainstream as to merit the kind of abusive comparisons to fringe groups as have been made here. OSC wants to keep the status quo on marriage. The KKK wants to lynch homosexuals. There is a difference.