First round picks have a fixed scale, second round picks do not but high profile players who slip to the second round usually expect more money and guarantees than most teams are willing to pay them at that point, which would seem to include the Spurs going by the 2nd rounders they actually did sign.
Could the Spurs have taken him without his consent? Sure, but what do you do when he doesn't sign the contract you're willing to offer? Do you really want to play hard ball with his rights, knowing he is a close friend of Wemby, and is also represented by Wemby's agent? This would seem like a big no no for most teams, and unfathomable for the Spurs.
Bottom line, if you want to treat the second round like the first, be prepared to pay them accordingly.