Fighters have, and should have, the right to reject fights. There are a lot of factors that go into a fight, and if you don't think one is good for you and the bad outweighs the good, you should be able to reject it. When you do this though you know that you are risking future fights as well. If you aren't willing to take on all challengers then I wouldn't exactly call out the champ. But lets take the Irving vs Silva fight. Irving was about as dumb as possible when taking that fight. If he won then he would have beaten a man not ranked at 205 and everyone would credit the weight difference to his win. He lost, so he lost to an unranked, and even worse he lost bad. If anyone can give me any real reason why he was even potentially smart for taking that fight I will rep you everyday for a month. You would have to come up with something proving he was getting an extra $100K or something to even take the risk because in that fight the risk vs reward was so off it is rediculous.
I also completely agree with rejecting fights that you think wouldn't be exciting. If I was a HW I would reject any fight with Big Timmy, at 205 I would reject a fight with Machida, and there is usually 1 or 2 in each weight class who LnP excessively or don't engange enough to pay off even if I lose. As we all say an exciting loss is better than a boring win.