Unlike patents, trademarks are not subject to tests for obviousness or prior art, and unlike copyright there is no concept of a public domain, although generic marks cannot be trademarked eg 'food' or 'clothing' are not trademarkeable terms because they refer to a class of goods. Nor can you trademark for functionality, eg you can't trademark handles on cups or wheels on a bicycle. And there are some specific exceptions like national flags, identities of individuals other than the claimant, or making false claim to a geographical region for agricultural products.

Otherwise, almost anything goes - you can trademark colors, logos, fragrances, and what-all else. The basic requirements are that a mark be distinctive - ie easily identifiable - and that it be used in commerce (at least in the US, because trademark registration authority derives from the Commerce Clause in the Constitution).

This trademark seems to have been properly filed, brought to public notice, and issued. It has no effect on the use of Pi in math books, computer code, discussions like this one, or most other categories, but it does apply to the use of the symbol on athletic/casual clothing. Sucks, but it's legit. Y Combinator could equally trademark the 'Y' logo that appears in the top left corner of HN for use in the context of business development, and someone would write a story along the lines of 'Investor Paul Graham trademarks the letter Y.'