No, it's clearly a website bug. There are fares allowing CLE-IAH-SEA (although the cheapest one for 8/10-8/12 is an H fare right now), but CO.com won't sell them unless you force it to.

Workaround: you can force the IAH flights to display using a multicity search: search for a four-leg flight CLE-IAH, IAH-SEA, SEA-IAH, IAH-CLE. This prices correctly.