Even if there is an area of 400NM of CBs - you cannot fly through. Flying through a core of a CB means certain death. Books say: fly around it at least 20 NM. You should never fly over or under them.
If they did - well, then that doesn't put a good light on them. Maybe they lost the radar (because of lightning), as someone already mentioned. It happened to me over Malaysia once and it's not a good feeling.
btw I flew 3000 hrs in the tropics and crossed the ITCZ up to 4 times a day. There is no big problem, you just have to do right thing.
Dani