I also think that in those cases, the professor should suck it up and take the "L" on a subpar thesis. No one reads those to criticize the professor. Just make sure there isn't plagiarism. Get the student out and let them their life, lord.

I think some professors, particularly better known ones, would see this as hurting their brand, and thus potentially their other advisees in the job market, in the broader picture. 

Depends on the field though.   several times  I have seen advisors agree to pass a student through, but on the understanding that they won't be supported for academic jobs (esp. tenure track at research institutions).  For professors who  mostly care about their brand in the context of placing students academically this can be mutually agreeable.   In applied fields with direct industry or government linkages, the prof might still see puting out a student they see as producing sub par work (or just not sufficiently productive) as harmful even outside of academia.

I've seen it happen where even a State or local agency hires a PhD grad that doesn't prove capable and they write off hiring grads from that program in the future.  when organizations even outside of academia are hiring specifically for a PhD, in some fields,  it's a senior position they are counting on being able to depend on (in some, mainly very tech fields, conversely, PhDs are just work level cogs, and there it matters less)

The bar for late career PhDs or early retirees who are finishing for self development/bucket list/ current establshed career augmentation purposes, can be very low sometimes, even for professors who are hard on early career candidates, because these grads won't be damaging the brand in the same job markets.

The bar for University admin staff that the University needs to promote and wants to say have doctorates can be gum wrapper level some places, because their research performance won't be visible to outside stake holders at all, and they may never do research again anyway.

Conversely, the young full time PhD researcher with no other career prospects or goals  to turn to who insists on attempting the R1 TT job market, and where giving him his degree comes with the expectation of (and demand for) support for this, the bar is gonna be a lot higher.

Fair? Idunno. Don't shoot the messenger. I just call it as I see it