I understand the question to refer to the income requirement on those applying for entry clearance to, leave to remain in or indefinite leave to remain in the UK as a non EEA partner or child of a British Citizen or person already present in the UK:
Q1. Does this discriminate against the poor? Yes, in as much as it will separate poor families, but not rich ones. However the test is acting as a proxy for the requirement that migrants must be able to support themselves and their dependents without recourse to public funds, which is a test applied to numerous other forms of visa, including for visitors, both by the UK and elsewhere, and is based on the view that countries have a right and duty to support their own citizens more than they do others. In other words the justification is that this is "an allowable discrimination".
Q2. The UK is currently a member of the European Union, and as such through interpretation of existing laws regarding freedom of movement doesn't issue or require entry clearance for EEA nations. As such, there is no point at which such a test on EEA partners could be applied. In principle if an EEA partner were found to be depending wholly on state benefits for more than six months, it might be possible to remove them, but I understand the legal situation to be complex.
Q3. It's unlikely it could be shown that the test is racially discriminatory, since it applies to all the people it's possible for it to do so, while Britain remains in the EU. When and if the UK finally renounces its membership, then it's possible this question could be revisited, however in the current political climate, it is more likely to be extended to all foreign nationals.