when a new operating system gets released it get ripped , when a new game is released it gets ripped and individual contents and model and textures from the game get ripped, when any software get released like phtoshop etc.. anything, it get ripped.
So are those all bad? the people who use ripped content , end users, dont rip it themselves in full because it is difficult to do that, someone else with the know how and lots of work does it and provides a tool to do it to end users .. this is easy to use and i do agree it can be said to be done with ease from that point of view but without those tools it is not going to happen.
Sketchfab developers are aware of such tools and are constantly working to make those tool not work. but the same can be said of those people building those ripping tools .. they are also constanty patching the tool to try and circumvent the actions of sketchfab developers. This is true for all software.
Internet security also has got huge teams of "good guys" working against the threats and malware of the "bad guys'. It never ends. Im talking about thousands of issues everyday you are not even aware of. Nothing digital on the internet is safe, nothing digital on your pc is safe. Even photographers and image sites used to use watermarks, but software became clever enough to remove those with one click, so now they simply only use really low res images as a result so work is not stolen. Captcha security on forms has to scramble some alpha numeric sequence in a way that software can not edge detect and predict those letters in a easy manner..why do you need that? because if you have owned a site or forum you will have to deal with bots and spam. it it rife and it exists.
The world is full of good and bad people everywhere, and self deluded entitled people who would not think of themselves as bad but rip everything everyday. It is not sketchfab's fault , they are not slack about security and it was not easy to any layman end user to actually rip anything. If the ripping tool is really good , it is not then the site or platform that is bad , it remains the ripping tool that is well made. Sketchfab in no way permitted the action of the said person The person who stole your work is a thief plain and simple.
You will never escape this ,it will always be a constant battle where ever you take your work and publish it. Safe today is not safe tomorrow... if you dont want your model stolen dont upload it anywhere in any 3d viewer that uses the model file/s in some way.