I agree with your general point (which I take to be, Apple is not seriously threatened by loss of sales to the US Government).
But remember, iPhones and MacBooks are quite popular everywhere, including US government procurements (e.g., https://37prime.wordpress.com/2012/08/05/nasa-mars-science-l...).
My experience is primarily DoD, which is the bulk of the government's spending on this sort of thing. For enterprise spending, Apple doesn't get much love, outside a general trend towards iPhones for "company" phones, but even that's rare, with people using their own phones instead of being issued one a lot of the time (they can get their plans paid for or subsidized if the phone is a requirement of the job). Labs often end up outside a lot of the enterprise type procurements and so have a bit more leeway in that regard, but while they spend a lot of money, it's still a drop in the bucket for Apple.
I do agree with your general point.
Your characterization ("Maybe some labs that wanted a unixy OS and, again, convinced their purchasing officer to buy a Mac Pro?") is a little off -- where I work, MBP's for laptop replenishments are treated exactly the same way as Windows systems, you just tick a different button on the order form.
I'm having trouble finding numbers, but I seriously doubt this. The reason that's true (or more likely true) for Microsoft, is Windows. The US gov't has massive site licenses for Windows and most of MS's software portfolio. Apple is used where in the US government? Some cell phones? A few public affairs offices that convinced their purchasing officer to buy a Mac Pro for video editing? Maybe some labs that wanted a unixy OS and, again, convinced their purchasing officer to buy a Mac Pro?
Per: http://investor.apple.com/secfiling.cfm?filingid=1193125-14-...
The bulk of Apple's revenue comes from outside the US. Perhaps the US government is their largest single customer (I still hold this is a dubious claim), but it is not essential to their continued existence. They would do just fine without those sales.