Probably because they weren't switched on at the time. An EMP (electromagnetic pulse) causes an overload of electrical systems through which electricity is flowing at the same moment. Consequently, switched-off devices will be less affected or possibly unaffected. In the case of military vehicles, EMP doesn't affect diesel engines (for loosely similar reasons that it doesn't affect firearm rounds), and it's likely that they were braced against electromagnetic disruption anyway. If they were affected, the military would have had plenty of time to fix the problem with their vehicles just as the van was. However, digital/computer memory systems (besides optical media) can be damaged, blanked, scrambled or reset to factory defaults from the interference of strong/nearby magnetic forces (whether fixed or fluctuating); so in regards to the camcorder and cellphones (even various kinds of walkie-talkie/CB/HAM units), it is definitely possible that the filmmakers ignored factual accuracy in favor of (suspense-invoking) artistic license. Also, strong/nearby electric forces (whether fixed or fluctuating) can cause batteries to be damaged or discharged, along with photovoltaics and piezoelectrics. Lifeforms can be affected in certain contexts too, as a matter of electrophysiology in general but especially in the context of needful magnetoreception (of Earth's magnetic field). Ultimately, the level of destruction or disruption depends upon the fields strengths (volts per meter, and teslas) of the pulse (the strength values being strongest at the spatial center of an event and weaker at distance), along with frequencies/bands of the signals and noise, as well as the amount of shielding in/around electric or magnetic devices located within the path/range of the pulse. For instance, a nuclear EMP is much stronger and spans more electromagnetic bands than a man-made, non-nuclear EMP, since the nuclear radiation (much like solar radiation) itself can actually cause material to heat up (melting, evaporating, decomposing or changing state in other ways).