Kind of amazing people are just waking up to this.
The Tawhid itself, an expression of the 'oneness' of god, is a direct reference to the Shahada–the primary of the five pillars of islam. It's the declaration of faith that all muslims must commit themselves to, where they state "There is no god but Allah, and muhammed was his final messenger". It's conclusive and meant to tell the world that they don't accept any other religion but their own. There is no room for any other interpretation. There is only one god, there will only ever be one god, and it's theirs–and if you challenge this by expressing any other opinion, they can choose to eliminate you with divine righteousness (and are convinced that they will be eternally rewarded for whatever action they take). There's no wonder why it's used as the symbol of the islamic state.