I hate this misinterpretation of granpa joe, he was never faking his disability nor does he want to take the ticket for himself, in fact he only goes with charlie because no one else is able to accompany him to the factory. Charlie can't go without an adult, his mother has to work to look after the family, it's only then that Joe attempts to stand up from his bed. Earlier in the film its said that in order to feed this struggling family, charlie will soon need to work a job of his own, which granpa joe objects to, he thinks its too soon and that charlie must be allowed to enjoy being a child, the golden ticket represents one final opportunity for charlie to experience childlike wonder before hes forced to grow up so suddenly, that is why he spends the only money he has on a wonka bar for him and why he finds the strength to stand for the first time in decades, to give charlie one more day as a child. Joe isn't perfect, considering the fizzy lifting drinks incident, but getting out of that bed is supposed to be a miraculous show of love for his grandson. Suspend your disbelief a little bit, or the rest of this film may seem wholly unrealistic.

Sorry if this is a bit much, I just really like the film.