Because the taxpayer, or the levy-paying utility customer, whoever is paying for the job, expects the job to be done properly.why should the taxpayer have to pay to have it flushed ?
It is not "free" in the sense that the supplier and installer of the new boiler are donating it as a gift.
The rest of us are paying for it to assist the more vulnerable and hard-up members of society. Very likely many of these are not even aware that they are being treated with contempt and receiving shoddy work. Those of us who are paying for it are being ripped off.
For example, if I live long enough, I hope to collect a small state pension that I have been contributing to for many years. It is not "free" and I should not feel grateful even if somebody pinches a bit of it.