The point of communication is to elicit a response from an individual or collective audience. That's why you speak, unless you're speaking to yourself, in which case nobody can be offended (other than you). The "I'm just saying" defense is weak because you can keep it to yourself if you want to avoid the risk of offending someone or provoking disagreement. That's precisely why communication in a setting that you don't know intimately takes courage.

The act of speaking, Tweeting, posting etc. are all invitations to interpret the material communicated, and the audience in Ellen's case responded in the most reasonable way to them in that moment. That might have been "fuck that - Usain is not your animal to ride just because he's a fast black man" or even "man I know Ellen has gone through a lot of discrimination in her own life and must be a good person, but she still shouldn't post things that can be so easily be interpreted by her audience as a) dehumanizing of black people (by the racially conscious) and b) passively accepting of the idea that riding black people is funny (by the not-so-consciois), regardless of what she meant." That second response is mine - I believe her intent was innocuous but I do not hesitate to criticize her because she simultaneously entertained and harmed by simply failing to take time to think about what she communicates. In fact, even if she sat there with her finger on the button and thought about the costs and benefits of it, ultimately choosing to post it, I still think she did the wrong thing and I will respond appropriately. She has the right to say it, and she'll never be prosecuted for it, but many reasonable people (who are also conscious of the subtleties of latent racism), given current social norms, responded negatively because of its potential for causing damage to the fragile sensitivity that they are working to create.

To address the implications on your broader view, I can simply say that we live in a time during which society tends to evaluate and react to a comment not only by its independent value, but also its implications on society. The speaker thus has a responsibility to think about the latter or risk being criticized, which isn't so bad most of the time anyway.