Is it ethical for a Professor to disseminate wrong answers online?
I have a strong negative opinion on this. 

In 2002, I joined a PhD program and was at the same level of computer science education as peers who had recently completed CS degrees at good schools. My last prior formal CS education was a master's degree that I completed in 1975.

I achieved that, as well as staying employable in the computer industry for over 30 years, by continuous independent study. As computer science kept changing around me I felt at times like the Red Queen in Alice in Wonderland: "Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!"

Not having answers to the questions in a textbook was something I could handle, though undesirable. If I had found answers on-line that conflicted with my answers I could have wasted a lot of time trying to resolve the discrepancy, including trying to contact the answer author to point out an error. 

Wrong answers to questions in a good textbook are particularly destructive. I put a lot of effort into selecting the books I use. In order to progress, it is necessary to attempt exercises that are a stretch. In some cases, it is difficult to check whether an answer is correct. Searching on-line for answers may be the best available resource.

The professor was, intentionally or not, sabotaging independent study.