tl;dr - for the same reason as any other coastline or complex border.

Also, it annoys me that the trail in question is advertised as allowing one to walk the entire English coast - but fails to mention Wales and Scotland are in the way (the trail is not contiguous).

▲

kelnos 4 hours ago | parent [-]

Assuming they specifically do advertise that you can walk the entire English coast, then it is accurate, as Wales and Scotland are not England.

	▲	bryanrasmussen an hour ago \| parent [-]
		unless they have edited they do not say Wales and Scotland are England, they complain the trail is not contiguous, which to be fair might also be implied by the fact that Wales and Scotland are not England but I guess I could see a scenario that allowed made the trail contiguous, which would still allow you to walk the English coast, but also of course allow you to walk some other parts that are not on the coast.