They did in fact make a course deviation, but the freezing up of the pitot tubes was not due to being in a "storm" per se, it was due to the character of frozen water droplets at altitude and a pitot tube that wasn't as able to handle that problem as other pitot tubes one could equip an aircraft with.
You will need to go back to threads 1, 2, and 3 and read about the in depth discussion on the radars and pitot tubes to help flesh out this answer.
Remember this: passenger aircraft flying at similar altitudes carrying hundreds of passengers cross through the
ITCZ every day and don't crash.
"They flew into a storm and crashed" is a red herring.
If you bother to read the history of this investigation and this flight, a few things become apparent:
1. The pitot tubes (all three froze up) were scheduled to be replaced in Air France aircraft, and some already had been, but had not yet been replaced on this particular jet. There was an airworthiness bulletin to that effect published a some months (years?) before this accident.
2. The pilots had already slightly reduced speed to account for flying through/near a storm and expected turbulence before they ran into that pitot icing problem.
3. The pitot tubes being rendered unreliable for a short time due to a particular kind of high altitude ice crystals made for unreliable airspeed.
4. The procedure for dealing with that at high altitude appears not to have been followed. That said, the UAS procedure has come under review since that accident. Many discussion on how to train for that malfunction have been recorded in the threads, on this topic. Use the search function on page on to find them.
5. Even with a less than stellar response to the UAS problem, a great many pilots who have years and years of experience flying that route will tell you that setting the correct pitch and power for the altitude and speed desired will keep the plane mostly level, at cruise, until the crew can sort through the malfunction and get the various systems back on line.
Point 5 is a pilot thing, not a machine thing.
While there was a machine malfunction, the pitot tubes taking time off due to ice crystals, most machine malfunctions have a series of procedures and remedy that the pilots apply to mitigate their impact, and to restore performance. The UAS procedure is one such remedy procedure.
The core pilot problem was in not applying the simple measure of flying an acceptable pitch and power while the NON-flying pilot would work through the systems to get them back on line. A contributor to that problem in this case seems to be the lack of hand flying time a lot of air transport pilots get at altitude in many fly by wire jets. If you don't practice something, your skills will be rusty when you need them. This is, based on the inputs of many professional pilots who post here, an industry wide problem.
The core machine problem began as all pitot tubes failing to function (even though there are three to provide redundancy), in part due to an already identified sub-par set of tubes being installed and not yet replaced by better tubes. Airspeed indication and input into the flight system is a crucial bit of information for both pilots and computers to use when flying the aircraft.
The radar don't really enter into it. The judgment call on how far to deviate from a given weather system is just that, a judgment call. Feel free to read the hundreds of posts on that element of this accident. The opinions among those who fly big jets for a living varies a bit on that point.
With that in mind, many experienced pilots note that some of the other intercontinental flights that night had deviated further around the weather system than AF 447 did. The analysis of the FDR data, once it was recovered, didn't show a significant amount of storm associated turbulence to have been a factor in the flight departing controlled flight. (Stall is a departure from controlled flight).
6. There were 32 incidents in the years previous to this mishap of unusual aircraft behavior and upset that were somewhat similar to, though not identical to, the malfunction the AF 447 crew ran into.
None of the others crashed, though some of them had bizarre altitude excursions that they had to deal with. A Qantas flignt (QF 32?) is one you might want to read up on for its similarity to the AF 447 accident.
Once again, please read the report.