It is selective enforcement. Read your CC&R's--they probably say someplace in there something like, "Just because we didn't enforce this previously, or against that person, doesn't mean we can't or won't enforce it against you or others, now or whenever we want."
And yes, it is part of living in an HOA.
Run for the board, read your CCR's, get involved.
since it's never been enforced--and maybe it was, one time--it may be not enforceable now, against that person. If that space is still common area, perhaps they should rent it. If the streets (including that area) have been turned over to the town/city, maybe you can get it enforced that way. Call your local zoning office to find out.
It may be that no one's questioned it previously. Get a few other homeowners and get it changed. Maybe it's time for a review of the CC&R's and enforcing them.
Do document, however. Take photos, etc.