It's the kind of kiss Obadiah expected from Sherlock, and it closely resembles the one he gives in return. Kisses, like everything else, can be tailored to match people. He kisses Tony affectionately, for example, because it's what Tony likes.

(He would have kissed Howard with heat, giving him a fight and letting him win it, hands in Howard's hair and body arched to accommodate his.)

But there's no point in tailoring a kiss for Sherlock, because kisses can be read, too. You can learn a lot about someone from the way they kiss. Obadiah uses that tactic when he can, and he knows that being honest with his kiss will win him more points with Sherlock than trying to lie.

So he, too, kisses with the intention of gathering information.