Is using academic writing services for a thesis considered plagiarism? Can they write in an original fashion?
There's a big difference between paying someone to proof read your thesis (generally acceptable, and sometimes encouraged) and paying someone to write original material from scratch (plagiarism). 

The way your question is worded (and from one of the comments about getting someone to "edit and write the thesis") it sounds like you are asking whether if you pay someone to write part of your thesis, will they do it well enough that it wont be detected by plagiarism software (e.g., the idea that people who write paid essays just copy and paste huge chunks from Wikipedia and random journal articles). My apologies if this is not what you are asking. But to state the obvious point, paying someone to write your thesis and contribute intellectually is plagiarism and unethical whether they do it in a rigorous or a sloppy way. It is plagiarism, because you are not acknowledging the source of the intellectual effort.

Surely if you are paying someone merely to refine your expression, phrasing and give general guidance on structuring, then there is minimal risk of plagiarism.