GWR are the ones who have created this ticket and set the restriction code; the nationalrail.co.uk website simply shows what their Pricing Manager has set up.
I suspect it's aimed at promoting contra-peak travel into Exeter but as you say, it doesn't work very well for journeys that involve a change. Equally the "change" rule is there because implementing the restriction any other way would be difficult, given the limitations of the industry time restriction data schema, and also because tickets could otherwise be used for commuting which GWR don't want.
