I completely agree.
Even though I admit that I tend to be ecoanxious and I do
as much as I can, avoiding heating and air conditioning whenever possible, I completely understand everyone has their comfort levels and forcing someone to bear heat or cold when they can easily avoid it is not kind or reasonable.
Like most situations, open dialogue is the solution. The HO needs to be flexible and not expect the sitter to be a clone of himself. The sitter should take a breather rather than get offended, and elaborate more about the desired temperature.
Maybe they’re just a couple of degrees apart.
Maybe the sitter needs an exact temperature only to sleep comfortably, so the HO could accommodate with a portable heater that can stay in the bedroom.
There’s also the possibility that the HO’s heating system is 40 years old and if it’s pushed too hard it will totally break and need replacement.
I love this! It reminds me of a listing I read long ago where the responsibilities section was a long, almost round the clock list of routine activities to be completed with the pet, it included drinking a cup of tea and sharing a yogurt with “Fluffy”. 
At a sit that I actually did, I received on arrival a pdf document, which was “complementary” to the WG and it mentioned something like “after the girls’ breakfast we go out to the garden. I then check my emails while I drink my first cup of coffee, it helps me get the call of nature “
No they really don’t. They have a duty to make sure that an unpaid sitter aka guest and pet carer is comfortable whilst they’re looking after their home and animals. The environmental argument falls over instantly if that sitter has flown to take the sit and the host has flown off on holiday. Yes we can wear a jumper in our own homes but we have a duty to look after visitors and make them comfortable. #treatthemwell