I think you'll find that the likelihood of a carrier re-routing is possibly related to whether they are part of the IATA ticketing clearence House.
Easyjet and Ryanair (for example) are not.
That means that if they are going to reroute you they must buy the ticket directly from the new carrier.
Airlines that are part of the Iata clearing house can simply issue a new ticket on their own paper to include legs on another carrier. IATA sorts out the billing for the various legs.
For example it's easy for British Airways to reroute someone via Lufthansa. They simply make the reservation via their own system and issue a British Airways ticket showing the LH details for the relevant leg.
When you book with a low cost carrier who is not a member of the IATA clearing house they simply can't do that.
I suspect that may well be why the "rule" is written in that way - those airlines have no mechanism to reroute in that way.