Not everyone, just those using the specific kind. The main issue is that there are thousands of potential hash algorithms to use in bloom filters. If we leave it open, all clients must implement ALL hashing functions just so that things don't break when they suddently face a hash that they don't support. Which, in the end, means that nobody can declare full support to the NIP because there are always new algos. Also, there is not really a huge gain from tweaking hashing algorithms for Bloom filters. We just need one that works and everybody will be happy. Just like we only use SHA256 on nostr events.