The legitimacy of it would be the same as England telling America "either you speak like us or you start calling it American" except in this case there was less time between colonisation and the current age where there's steady traffic of information between the two so the differences aren't as significant.
And it's not just Brazil, Portugal was a great coloniser and until "recently" still had a considerable amount of colonies (in it's heyday the Portuguese empire rivaled the British if not surpass it). The Orthographic Agreement started as a necessity due to printed works and the idea behind it was to make a unified language based on Portuguese but with influences of all the colonies, that way even if spoken differently one copy of a printed work would be understood (and more importantly economically viable) for all regions.
