As
@bhg469 suggests, it's all about usage versus ride capacity.
Universal has 3 onsite resorts offering 'free' Express Pass. Those 3 resorts have about 2400 rooms. Assuming 3 guests per room, that's about 7200 people using Express Pass even if the hotels are full, only a fraction of ride capacity. That's what allows Universal to sell Express Pass at a premium to offsite guests. Universal has a lot of high capacity attractions and excess ride capacity.
Conversely, all WDW guests get 3 FP+ and well over half of ride capacity already is allocated to FP+, resulting in significantly longer Standby lines at WDW. Consider that WDW has over
26,000 onsite rooms. Assuming 3 guests per room, even offering 1 extra FP+ selection to onsite guests would strain an already overburdened FP+ system.
In order to support more FP+ for onsite guests, WDW needs to add significantly more ride capacity at 3 of its 4 theme parks.
I think, at least in part, the indignation is a result of already very long Standby lines at WDW, lines that seem to be shorter at Universal.