You wash underwear in the washer, work clothing, socks etc do you worry about those things transferring germs to the washer? Of course not. You use washing detergent every time don’t you, that’s what detergent does same as when using a detergent to wash dishes. If you keep your washer properly maintained with no standing water left in it, run a clean cycle as recommended then there is absolutely reason you not to wash your mat as long as the instructions on the mat itself say you can machine wash it.
No they added that button specifically to tease and confuse people!
You know you are supposed to at least skim the manual of an appliance before using it don’t you? If you had you would see that the cold setting is a viable working option of your hairdryer as blasting the cool air through your hair when you finish helps set the style it also helps cool your scalp so you don’t get that immediate layer of heat prickles when you finish