By putting your grass clippings into the black bin, you are sending your waste to be landfilled, where during its breakdown, it'll produce gas and leachate which has to be collected & treated, therefore making landfills more difficult to manage.
The Plan going forward is to divert all organic waste away from landfill into green waste and other composting facilities, as well as subjecting it to other Mechanical and Biological Treatments.
This way the waste can be processed to produce compost, gas or fuel for incinerators (!!).
Its a bit daft to send it to landfill as you pay through the nose and you are burying something which is quite amenable to simple processes designed to recover its value.
Legally, there may be local bye laws requiring you not to put in into your black bin, but you're unlikely, right now, to get caught, though it does tick (ahem) the operators off.
If I may digress a little, there is also a new ban on gypsum based materials going to landfill also (plasterboard) because of the fact that when organic material (such as grass clippings) break down in its presence, it generates hydrogen sulphide gas, which is fairly nasty stuff, occupationally and environmentally speaking.
Best thing to do with your own grass clippings is compost it yourself, its easy peasy really, or else put in in your brown bin, absolutely acceptable, or else transport it to your local civic amenity centre and into the green waste receptacle, or otherwise, and I'm not condoning this, just lash it on a piece of waste ground out of sight, and in relatively neat fashion (without other contaminants), and it'll just return itslef to good old Mother Earth.