There is one huge difference:
With 'automatic tipping' you're no longer rewarding the person for doing a good job.
Everything being normal, I'm a 20% tip guy.
If you suck - it's nothing.
If you're awesome, it's likely to be above 20%.
If you force a tip on me (18% or 20%) that's where it ends with me. You get the forced gratuity and nothing more. You can do a super awesome job and because of that stupid policy you'll get whatever they forced from my pocket.
Allow me to make the call and, normally speaking, it's 20% from me.
I'm going to sign up for TIW when I go next week but I do admit that the auto-tipping irks me.