They don't decrypt HTTPS. The NokiaBrowser (former OviBrowser) is a "proxy browser", a different application that talks to Nokia servers and these servers than establish a regular HTTPS connection. The same as Opera Mini. There is one more layer of abstraction.

Do you trust your phone operating system? How do you know it is not capturing your data when you access a bank website? Same issue here.

The one difference is that the Nokia servers might be a bigger target for hacking, instead of hacking individual phones.