The British government has a duty to protect the people participating in and watching the Games. They thus have a very pronounced, legitimate say in matters of SECURITY. But hateful speech (like Nazi salutes and racist grunts) doesn't truly represent any security risk.
No. They didn't fine the guy on the basis of the notion that his "speech" constituted a security risk. They fined him on the basis that his KIND of speech was deemed hateful or otherwise worthy of suppression on a NON-security basis.
"This is about respecting their opponents, it is about respecting the games, the Olympic values, and it is a celebration of friendship between people from all over the world." -- quoting, evidently, International Basketball Federation secretary general Patrick Baumann in the article cited in the OP:
News from The Associated Press.
Here's an interesting legal analysis from the British "Crown Prosecution Services" web site.
Racist and Religious Crime: Legal Guidance: The Crown Prosecution Service