Hang on now.
This is a complicated issue in some respects.
When the US turned off SA, it meant that the GPS signal to a certain accuracy would not be blocked. ie the key code to the algorithm that a civilian GPS uses, would not be blocked.
So, actually, all of the GPS broadcasts have been encoded, it is your receivers algorithm that can decode the signal to a certain level.
The GPS signals have always had higher levels of accuracy.
Given that,,
There is no way for ASA to limit any GPS broadcast, because first off, they do not own the broadcast, nor do they have any way to add a restriction in the equipment.
Next, the GBAS system is a worldwide standard, the broadcast uses ARINC 424 standards, there is no specific encoding in the signal that is available for restriction.
As far as a subscriber base, the ability to use GBAS, if they wanted funding for it, would be based on the clearance to use the procedure, not the ability to receive a signal.
The GBAS procedure itself could be subscriber limited, much the same as tailored RNP is now, you dont pay for the procedure, or are not cert, it is not in your box...there are plenty of RNP, and RNP to GBAS final approach procedures that are not in the public realm, but are used by the airlines/entities that have paid for them.
As an example, I have designed many tailored RNP-AR procedures ARR and DEP, for YMML and others airports in AUS, that are not avail as public charts, but are used on a daily basis by the airlines that have paid for them.