For that to be true then these guys would be hitching a ride and there should be more guys inside.For me the explanation is much more simple. They did it becouse they are quite far away from combat zone and it's much more pleasant to take a ride than walking in the mud
The truth is that it is safer from two threats sitting on top and they are the biggest threats to armour today.
Threat one is landmines... if you are sitting on a seat in an armoured vehicle and a landmine explodes beneath the vehicle there are a lot of back injuries to crew and passengers. If it is part of an ambush try getting out of an APC with full gear with back pain.
The second thing is HEAT warheads, either from rocket propelled grenades like RPG rounds of ATGMs, which turn the inside of an armoured vehicle into a pressure cooker. The liquid hot gas of the metal liner will destroy lung tissue and burn exposed skin and generally fk you up. Sitting on top you might get light fragment wounds or even be able to jump clear before the rocket hits.
Another reason is that it is stuffy and dusty inside armoured personel carriers.
Look at photos of any APC that is not in direct contact with the enemy and you will see soldiers all round the world do the same, including on M113s in Vietnam etc etc.
Regarding protection from IEDs, if the IED is using a HEAT warhead to penetrate armour then sitting on top is better than sitting inside for the reasons stated above. If however the IED is just a really big bomb to take out the vehicle with blast or fragment damage then it is usually safer inside the vehicle than on top... if the bomb is big enough (like a wired 500kg aerial bomb) then it really doesn't matter whether you are in or out of the vehicle.