This can be taken one of two ways: On one hand, this could be taken as a sad move, where he just wants to keep sitting and collect his money. I don't think that's the case, as I think (hope) it's more likely that he's just doesn't feel ready and feels bad that he'd be taking the spot of another player, and for the good of the team, he'll continue to work his way back.
At least Argenis Reyes gets to stick around.
Update, 5:51 pm: Here's an update from Rod Boone over at Newsday:
According to Manuel, because the team is playing so well Castillo did not want to affect the team chemistry. So he won't be activated today and there's no telling when. Manuel said it wasn't a health-related issue and Castillo will travel with the team to Philly. He will be activated before the rosters expand Sept. 1.Valiant move, Castillo. I like this. A lot.