The CDC guidelines aren’t about prevention but a trade off between prevention and economic activity.

You can very well be contagious during that period, but it’s too much of a burden on the system to wait until you are definitely not contagious.

Up here in Canada when they changed the rules and had the big press conference they even explicitly mentioned that it was in part due to economic reasons and it would allow some additional spread when asked directly.

Edit: and they even recommended wearing masks to work for a few more days afterwards just in case, but of course no one did.