It's the decision of the Police that to go and make an arrest is likely to cause worse disorder. Handling a crowd, particularly a crowd that has a sense of unity, or comararderie with each other, is different to dealing with small groups or individuals.
She was protesting against segregation in every day life. If you want to protest against segregation for safety and keeping the peace, in large crowds, buy a ticket in the away end next time you are at the Amex.
Copper is trying to keep the peace, not create Apartheid. It's the same reason you may be marshalled a bit at football matches, for being visibly Brighton.