I would assume it's offered for different reasons, some do it just to get more profile views, some do it because they feel lucky, some actually have an incurable std and don't care or know they will rotate their profile/disappear long before you get your test results. Then there are some who seriously lack education on STDs or think everything can be fixed with a bit of medicine.
The bottom line is you can't tell people what to do and shouldn't try. As long as you bag up then that's all that matters.
God help you if you believe anyone that says they are 'clean and tested' and will take you bare for and extra 20 quid