Rumor has it the issue is residuals but on the face of it that doesn't make sense. Why? Because you would expect residuals on a streaming service to have somethign to do with how many people watched the show.
If that were the case, there'd be no reason to remove it. So the obvious answer is that's not how it works. My guess is that HBO is paying residuals for having the show available regardless of whether or not people watch it. The problem then is that not enough people were watching it. That makes sense given it was also cancelled recently.
Another potential reason is the way residuals are structured. Writers' residuals decay over time. That is you might get $1000 for writing a show that you wrote a year ago and is broadast/streamed. If it's a 10 year old show you might get $100 for the same audience. Residuals for writers for streaming service is one of the big issues threatening to boil over into another writers guild strike.
Given that though, HBO might be removing Westworld while residuals are the most expensive and it could come back in a couple of years.
Yes, there's a lot of speculation here.
I agree it does feel like HBO is breaking their viewer expectations by removing one of their own originals.