Presumably that the water bill (for tap water) was priced to cover both tap water provisioning and sewage works. But people using (free) rainwater to flush toilets ruined the pricing model, making the tap water price go up.

I honestly don't see the problem, it's probably still worth it (because society still needs to provide less tap water and saves there).

GP is partly right. Most of the cost of sewers is fixed cost: employee salaries, building and maintaining X kilometers of sewers, etc. Some is variable: chemicals, but a small part.

If you, a single person, cut your water usage in half, you pay half as much. But if everybody uses half as much, the system still needs about the same amount of funding. So now you double the per-unit price, and everybody pays the same they were before spending money on water saving features. In this case, even if each person used half as much water, the total water needed isn't cut in half because the sewers need more water to function.

(Also, water isn't "used"; most of it's transported, cleaned, transported, dirtied, cleaned again, transported)

Perhaps that sewers need a certain volume of water flowing in order to function correctly. If that water does not come from toilet flushes, etc then they pump water into them to compensate.

The conclusion that saving water is greenwashing is just wrong.