[recycled sixtie]

No pilot is forced to fly in to a thunderstorm. If there are thunderstorms hanging over the airport it is the pilots discretion whether he/she takes off into it. An ATC controller cannot force the pilot to fly.

The same with landing. Nobody can force you to land. You can hold, divert or land.
There is a likelihood that downdrafts in some thunderstorms can exceed the climb performance of modern jet aircraft. So if you want to land or take off into or even near a thunderstorm it is a crap shoot.

Flying with a serviceable weather radar is mandatory if thunderstorms are forecast.
To do otherwise is negligent.