There's a constant struggle between allowing free speech versus restricting it via trademark laws. In general, they want to make sure companies can talk about and mention their competitors by name and not be silenced by trademark lawsuits, while still ensuring the trademark isn't being infringed to the point of confusion. "Amazon Elasticsearch Service" sounds an awful lot like they have permission to use the Elasticsearch brand, while simply referring to it in parentheses indicates it's a competing service to the actual elasticsearch.