Actually the music for the title crawl and several other pieces from the OT are available with complete parts, ready to be performed. I've looked at them from time to time for reference while orchestrating the FotG music, but unfortunately only some of the popular pieces are available (Imperial March, Throne Room, etc.); the complete score is most likely locked up tight in the Lucas archives. Fortunately it's usually not very hard to figure out what's going on as the style is pretty transparent, at least in ANH.
I imagine the suites are probably pretty popular with highschool and other small orchestras, so it is entirely possible that whoever sequenced the version you have just copied all the parts over from the C score rather than doing it by ear, though I don't think it actually matters in terms of copyright infringement. The tunes themselves are owned by Lucasfilm, so any iteration of them (even something as stylistically foreign as the Meco disco anthem) has to pay royalties. The only ways you can get around that are as a parody (which we are clearly not) or as "fair use" quotation in an otherwise original work. An example of this would be someone referencing a well known theme as part of their solo on a jazz record.
I have actually done a fair bit of this (using the OT themes or fragments thereof in unfamiliar contexts) in the FotG score, but that's as far as I want to go with the John Williams material seeing as I've also completely ripped off his orchestrations, treatment of motives, and any other element of musical style you can think of. Actually, the only reason I've even gone that far is that JW did so much obvious ripping off of other composers himself.