I have no idea why the vehicle was on the C-17, but am equally bemused as to why it’s presence would be seen as bad publicity.
There are literally dozens of aircraft arriving and departing. Many going in will be carrying tons of freight such as food, ammunition and vehicles - either for use on the airfield or by SF outside it. Many will not recognisable as military for obvious reasons. Much will abandoned when the evacuation reaches its end, what can be recovered will be.
There have been numerous photos of flights departing with only a handful of passengers. That is because the bottleneck is getting people onto the airfield and cleared onboard (we have already had one pax who turned out to be on the UK “no fly” list).
If there is a question it is why you think space not required for evacuees should not be utilised for vehicles, freight or other materiel but should be left empty?
Apart for
PR purposes of course - but that’s not what the movers and crews turning the aircraft are there for.