Everyone does.
Touchwiz isn't just the launcher, it's the entire fork of Android that Samsung uses.
Samsung changes just about everything google provides in Android, but does it JUST enough to still be official Android and support google services.
So far, I don't think there are any custom, non-touchwiz roms for the S6. Though i'm sure they'll come eventually (They better).
I personally cannot stand touchwiz and the changes Samsung has made. the second Cyanogen is out for the s6, I'm switching. It's not so much lag (there's a little noticable if youv'e come from a nexus device). But the changes to the UI of Touchwiz frustrate me to no end.
The Notification drawer is ugly, and wastes 1/2 your available display real-estate because of the pulldown for quick toggles. (Stock Android has a different gesture to bring down quick toggles, leaving your notification space 100% for notifications)
the rounded buttons also don't fit the rest of the Material Design theme. The Options menu is more convoluted, and options are burried behind more clicks. the Lock screen is dumbed down and doesn't display Album/media art from whatever media player you are using. You cannot change several default apps, such as what clock Touchwiz defaults to using. The flipboard style page on the default launcher has a 2-3 second lag when switching back to your homescreen. There are default apps you cannot remove at all, or disable.
And resources it takes up are just pathetic. fresh, brand new S6 takes approximately 1.7GB of RAM of the 3GB. in comparison to Stock nexus which is < 1gb. for filespace, Touchwiz uses 6gb of your storage, stock uses <3 gb
simply put, Even though 5.0 touchwiz is a dramatic improvement over look/feel of the 4.4 variant, it is still a ridiculous, bloated, laggy mess that in no way accurately represents Google's own Android.