Originally Posted by KVS

In theory, as per the IATA rules, any member airline has an obligation/duty to re-route you in case of irregular operations, regardless of the fare you are on. Any additional costs that the airline may incur if it has to re-route you on another IATA carrier, or, if necessary, in a higher class of service, is not a consideration.

In practice, most airlines will do their best to avoid such costs by choosing the least expensive re-route option. The "because of the way [your] tickets were issued" excuse is an example of just how creative airlines can be in situations like these.