E-prime forbids the use of the verb 'to be', in any of its forms. Advocates of the style of 'E-prime' often maintain that the use of 'to be', when applied to two noun phrases, falsely suggests that the two noun phrases are the same thing, when in reality they are not.
This can only be the case when the meaning of the verb 'to be' is incorrectly understood. When one says "(noun phrase 1) is a (noun phrase 2)", one does not mean to say that "(noun phrase 1) and (noun phrase 2) are exactly the same thing, and also (noun phrase 2) and (noun phrase 1) are exactly the same thing". What one actually intends to communicate is that "there exists a set (noun phrase 1), and a set (noun phrase 2), and (noun phrase 1) is a member of the set (noun phrase 2)".
To use mathematical terms, "A is B, and A is C" does not mean "A = B, and A = C" (from which the deduction "A = B, A = C, ∴ B = C" would be vaild), but "A ∈ B, A ∈ C" (from which we can deduce nothing).
To deduce that "Statisticians are male" from the statements "John is male", and "John is a statistician" is incorrect. It is, in fact, a common logical fallacy, the fallacy of the Undistributed Middle.
To be subjective for a minute, I believe that E-prime's proscription of 'to be' is too simplistic to achieve its goal of avoiding subjectivity stated as fact. "Tony Blair hates the British public; he wants to give all our jobs to asylum seekers", for instance, is subjective, stated as fact, loaded with prejudice, and a perfectly valid E-Prime sentence.