There's absolutely nothing wrong with this. The blocker does it because just deleting the object would let obnoxious websites to detect its presence and punish users for it.
The better question is why their checkout completely breaks because of this. Their reliance on Google spyware is 100% on them and they are losing sales because of it.
Analytics and ads are annoying, I get it. However if you use an automated script to clear them out, be aware that some things might brake.
If Ublock Origin devs think they need to replace GA with a dummy to stop tracking they should've put a copy of the original interface with empty functions into it so the flow doesn't throw exceptions when the app tries to use it thinking everything's fine. Those scripts are publicly available and aren't a secret.
They shouldn't require tracking to succeed in order for me to buy the product. If they want tracking, sure. But be resistant to it erroring out. Don't let errors in 3rd party tools prevent your user from getting their core goals completed.
The same goes for client apps. Don't crash the app if it fails to log to a file. Don't crash the app if it can't sync your cache. Etc, etc. Don't let these unnecessary conveniences get in the users way.
When two independently-owned systems on the web break each other, "who needs to fix their stuff" is a question more of social networks and business politics than technology. TeeSpring's "fix" could be to pop a banner that says "WARNING: uBlock Origin breaks this site and we can't test for that."
That'd be great. Then I, as a user of uBlock Origin, can nope-out of the site before wasting too much of my time and the site's resources.
And there are sites that will throw up a banner that says 'Adblockers might break this, if you have problems disable your Adblocker and try again' which is pretty effective. Funny enough, in my experience, sites with that banner tend to work with uBO enabled (probably because they're testing it).
Tracking should rely on the tracking service, not the checkout process
