West Ham don't have a history for blocking transfers so I don't see him worrying too much. Spurs, Levy in particular, are a different kettle of fish. No Spurs player or agent will trust their word going forward, which causes it's own problems.There is a need for one if the player has any desire to potentially play for a "bigger" club in the future, and they don't want the club they are contracted for to provide too steep an obstacle for that to happen.
I'm sure Kane was kicking himself he never had a release clause for his final, lengthy Spurs contract.