I think that Shia LaBeouf's apology for plagiarizing was sincere and that he really meant it. I think that he understood what he did was not very cool and that people were not happy with it. I think he felt bad that he did it or maybe didn't actually see it that way at first.
I do not believe Shia LaBeouf's apology for plagiarism in sincere. He simply got caught and generally people are quick to apologize but generally don't mean it. I think this is common among humans and it is evident with the amount of apologies heard in today's society. Saying sorry is common, meaning it is rare.
Shia LaBeouf is insulated by his celebrity status. I doubt he even understands the ramifications or issues at hand with plagiarism. He apologized, but has plagiarized repeatedly. I wouldn't be surprised if he didn't even understand what plagiarism meant. Without any punishment, I doubt he cares what this means and who is upset with him.
It is almost impossible to believe that someone such as Shia LaBeouf could have thought his apology for plagiarism, itself plagiarized, would not have eventually been revealed for what it was--unoriginal and insincere. The similarities his recent actions bear to those of Joaquin Phoenix' for his performance art film "I'm Not There" are striking, and perhaps the most likely explanation for his purposefully insincere apology.
Shia LaBeouf's recent behavior seems to mimic that of Joaquin Phoenix a few years ago when he was acting weirdly for a documentary movie. LaBeouf's recent antics include being part of an art exhibit and wearing a paper bag on his head during a movie premier. His apology isn't sincere because the actor seems to be acting like a druggie.