Okay I have taken considerable amount of time to examine the issue because for me it was not a bother at all till I saw your question. When I look at the metadata of Xml Site Plugin verses that of WYSIWYG Editor plugin I see the reason why you see this situation. I installed one of my plugin and saw it getting enabled which I cant disable which I found intresting. Okay the plugin manager checks if a plugin has a "load_order" in its metadata. If it doesn't have like the case of the WYSIWYG Editor plugin it enabled status is true and permanent (can't be changed).
- Disabling a plugin is not optional when
- load_order is set to before_db_init in the plugin's metadata
- or load_order is not set at all in the plugin's metadata
- or plugin is missing metadata which is the case with most plugins.
- Enabling/Disabling a plugin is optional only when
- load_order is set to after_db_init in the plugin's metadata.
To prove this: add the following line in xml-site-plugin's metadata
then reload Plugin's page.
sitemap plugin can not be enabled by default.