status reason for the campaign activity is linked to the status ("state") - and for each State, there can be 1 or more status reasons as valid options
If the state = inactive there's typically only 1 status reason (also 'inactive') and unless you change the state, you won't be able to change the status reason.
Review the value and the configuration and see if the state of the record has only one valid status reasons - if so, that should clarify the issue.
Additionally - Campaign Activities have functionality associated with them for distribution / resolution - there may be a functional reason for CRM to not allow the change by updating the status reason directly.Scott Sewell, CustomerEffective | http:\\blog.CustomerEffective.com | Twitter:@ScottSewell