the reason Trek specced a Tiagra front on the 520 is becasue that is Shimano's BEST full size road derailluer that is still 9 speed chain compliant. Shimmy's 105, Ultegra, and DA front derailluers are now 10 speed only - narrower cages for 10 speed chains.
Not that it's a big deal, I imagine you'd just have to trim the front derailluer more with a narrower cage and a nine speed chain. Shimmy's rear road derailluers are both 9 and 10 speed complaint, not an issue for the Trek 520 anyway, as it uses a mountain cassette and mountain derailluer.
Same with the front crankset being only Tiagra. Shimmy specs out the new, external BB, higher end road cranks as 10 speed only, although 9 speed chain works just fine on the higher end cranks- i'm running external BB, "10 speed" 105 and Ultegra cranks with 9 speed chains, no problems.
Yes, Shimmy barends shift FRICTION only for the front. allowing easy trims.
I'd bet Trek specs the 520 with a road crank because they sell more for unloaded, supported touring and city bikes versus a 'dedicated' touring only bike. For light touring loads, sagged tours, CC tourers, the road crank makes sense.