They're in the same division?
And they'll be playing with their full teams during the season. If Ball gets shown up by Fox during the regular season they can brush it off because the Lakers should still walk away with the W. Even if Fox dominates Ball during each regular season meeting, the talent discrepancy between the two teams is enough that the Lakers should be able to beat the Kings easily. The win will be all that matters. Meanwhile shutting him down after a slow start to Summer League could have wrecked his confidence and brought a ton of negative press on him before the games mean anything.
The Kings should be one of the bottom 5 teams this year and it's kind of sad because they actually tried assembling a roster to win or compete, they're not built as a tanking team. During the regular season Fox may not even up guarding Ball either, depending on the lineups that are used. They'll probably want him guarding Clarkson or KCP to slow down their scoring instead of Ball, who isn't much of a scoring threat.