You’re misunderstanding what a todo is in Joplin. A todo in Joplin is a not with a checkbox in front of it that can have an optional todo date (complete with an alarm). This is a frequently used feature of Joplin and will not likely be removed.
What you’re looking for instead is 
this button. Or on mobile I believe autocompleting checkboxes will work eventually (which should answer your other forum post)