I believe it is ethical to use the custom essay writing services. You do not still anything, you pay enough fair price for the service you use, you simply delegate the part of your work and can concentrate on what really matters to you. You are free to get an outside help and delegate your writing assignments to professional writers same as if you would get your watches fixed or car repaired by third party experts.
During my study I had several courses that I considered to be absolutely useless, I loved my major and make all writing assignments myself, but I hated Compositions, where you need to write some blah-blah-blah about your personal experiences and make 5 revisions polishing your work. I always ordered papers in such cases and haven't experienced any ethical problems with that. Instead of distressing myself I could concentrate on what I really liked.
I did some research to find out is it legal to buy essays, term papers, etc. I personally have never experienced any legal problems with using such services. I did have problems when some "trustful" custom essay writing services provided me with plagiarized papers, but it never caused any legal problems.
From what I have researched I found that it can be a problem for companies that provide the service, especially for paper mills. Some states have statutes against the sale of a "term paper, essay, report, thesis or dissertation" to students. But you, as a student, cannot be detected if you use trustful custom essay writing service and you will not have any legal problems, they keep all information private.