I also would guess they thought Mayer was(is) going to end up at 3rd and if they took him 1st it had to be to stay at SS. They also made big SS signings with Santana and Osorio.
In fact as you note, you have to count internationals. TBF, they've gone after a lot of position players internationally (as above), and considering that's where something like 30% of players are coming from, it's incomplete to just look at the domestic draft and say they are necessarily overly pitcher centric.
Pitchers are half the roster in today's game, in the long run that equates to half the picks.