I guess the main reason the plugin is enabled by default is to promote it. AI tools are becoming an important part of the development workflow, with a majority of developers using a chat LLM or Copilot (https://www.jetbrains.com/lp/devecosystem-2023/ai/). So the functionality offered by the plugin might be interesting to a large part of the user base. A non-bundled plugin would not be discovered by most users. And for organizations it looks like the AI assistant is disabled by default.
Regarding the direction - LLMs are usually used as a service right now, so there's not much choice. At the same time, JetBrains is also working on local smart completion workflows like https://plugins.jetbrains.com/plugin/14823-full-line-code-co.... Hopefully at some point there'll also be an option to use a local LLM with the AI Assistant.