I have checked this out in 7.4, 2013 (7.5), 2014 (7.6) and 2016 (7.7), and it acts as you describe. I also checked the design specifications and it says that it SHOULD be greyed out. From the design document:

"One of the fields in the selection grid will be fiscal year, and one will be fiscal period (but not limited to). The “Type” column for the fiscal year must be set to “Equal To” and cannot be changed. This means that only one year may be printed. The user may select a range or multiple periods to print within the year selected. The Period will default to “Current”, and the Year will default to the current Fiscal Year."

So if your system somehow did not do it, then something within your system was likely doing it, against the design. Probably not the answer you were looking for, but it should indicate that the behavior you describe is by design. This type of intentionally limiting a particular field to a specific value is not unique within Sage 500 ERP.