I'm let to believe that aerial photos taken by US government agencies (would the military come under that? I'd assume so, but don't know - hopefully someone will) are classed as paid for by the public purse, therefore they are available for the US public to use.
Unfortunately, that isn't the case in the UK as a number of organisations have recently discovered to their cost... Just because you can view aerial imagery online doesn't mean you can use it for anything else and, having just had their funding slashed by Her Majesty's Government, the Ordinance Survey are getting more vociferous in defending their income on what they do have.
The rail simulation crowd had the same debate not so very long ago regarding elevation mapping data, which led to the OS going after a number of sites and developers then watching the upload sites very carefully for usage of their property. I'm led to believe by someone who currently works with geospatial data that I asked for advice regarding obtaining it legally that this is far from uncommon and they don't tend to go after individuals first, but go for the distribution sources - i.e. where you upload the file to. Hence my comments to Falcon above to be careful because these companies have both the clout and will to defend their claims. If the Copyright holder is the Ordinance Survey, then the Copyright belongs to the Crown, which appears to have exceptionally deep pockets!
Finding out who owns aerial photographs is normally easy - both GE and Bing Maps state it at the bottom of the screen when you view them in a browser. That isn't shown in SBuilderX, though, so you'd need to look at the same source in their own client software to see who the owner is.
The OS may produce and procure very good data, but they do like charging a fortune for it.
BTW, on topic, I've done a WW2 Dunsfold in the usual way I do, but it's WW2, not modern, so wouldn't clash with anything done that has Top Gear track markings on it.
Ian P.