The 7/11 robbery doesn't make any sense to me, why would they go and do this after their faces get plastered all over the news as bombing suspects?
Also, at this point, I wonder if Dzhokhar is held up at a safe house somewhere? It makes me wonder if he is staying at a friend/ realities place just to lay low.
I think it was more of a case of them being spotted and then a chase ensuing. I doubt they would seriously go and try and rob a place but who knows.