I don't really see how the same applied to Wesley. Both Holland and Lilah were both full fledged subordinates of the senior partners; they worked for them so they signed standard contracts to do so. Wesley (and all of Team Angel) were supposedly given a no strings attached offer to run the L.A. branch of Wolfram and Hart. I don't know about anyone else but signing away your soul sure seems like a pretty big string that none of the characters would have agreed to. Wesley was not a standard employee; he was offered the chance to run a department in W&H with no caveats. He more so than anyone else (except maybe Angel), knew about the perpetuity clause and would never have even been tempted by W&H's offer if it meant his soul. Shoot, he'd also try to dissuade everyone else from signing as well. Don't forget that in season 5, the only string involved with them taking over W&H was that if they don't compromise some of their morals and do a little business, the branch would go under and they'd lose their resource.
Not the mention that it also means that the senior partners own the souls of every Team Angel member, including Angel. To me, it seems like a gigantic plot hole that they put in just so Wesley would still be in the series and ticks me off because he's probably my favorite character.