You almost found a bug but no... Keep looking.
The reason you cannot tick it is because you have no custom useragents set, go in to "Edit Custom Useragents" and add one of your own useragents to the default list and click Save. Now... you can tick Use Custom Useragents because you have a custom one and not just the defaults.
No custom useragents = No tick option
honeyandthemoon, which harvester the single threaded or multi-threaded? Try the opposite to what you are using, they use different Google search pages.. I have a hunch Google is changing something or testing their design/layout in certain location or percentage of people..