It's not '0 load', involves a capital expenditure for AT&T and while your call is going over WiFi it is going through their switching network -- it's just getting there a different way. For users with unlimited calling plans, it doesn't matter. For those with minute-plans, it comes out of your 'bucket'. While it may lesson the load on AT&T's cellular network, it also allows customers to experience clear calls in areas where signal quality may be low, frequently dropped calls, or no calling at all. If this bothers you, shut it off. This isn't AT&T offering a low-rate VOIP service that >only< works over WiFi, they're enhancing your cellular service.
There was never going to be, and there isn't now, a 'reocurring hd-voice fee', no idea why you'd say there is.
A benefit of WiFi calling is that it also means if you're in an area where WiFi Calling is available, you now also have
AT&T's HD-Voice, their branding for VoLTE -- a capability of 6/6S iPhones.
Sorry, your attempt to suggest some sort of 'criminal' AT&T 'WiFi-gate' is baseless.