I’m not a FoxPro user and so maybe these are single-user/single-host installs, where the user can only destroy their own data, and permissions are a pretend feature. In which case, carry on.
There's different needs for a permissions system.
You're thinking of it as "lock to the main entrance of a maximum security prison". Think of it more as a lock on a bedroom door: it's not there to withstand a SWAT assault, it's there to keep people from accidentally walking in.
All local applications (which were all applications in the era we are talking about) with permissions were sold to companies on the understanding that there was no real security.
Hell, even the networked products at the time had no real security :-/
The rub here is that FoxPro is an in-process database engine and accesses its data files with the security context of the user running the application. It doesn't have separation between the database engine and the UI like a client/server database would. Think of it like SQLite or BerkeleyDB.
Architecturally users who use the application need full read/write access to all the data files for FoxPro to work.