For the same reason burning wood is green- as long as you are regrowing it.
So - if you're burning timber from a forest which is sustainably managed (so you're renewing wood at the same rate you're using it), then there is no net carbon released.
That would be true if carbon dioxide was the only greenhouse gas. But it isn't.
Burning biomass could be sustainable if we make sure that new biomass is grown at the same rate as it is burned. I have yet to see a document assuring me that is the case.
In any case burning biofuel is not greenhouse gas neutral. And hence not green.
Because biomass will naturally released CO2 when decomposing
Add that biomass usually releases CH4 (methane) which is a considered a much more potent greenhouse gas. Even simply burning the methane to CO2 -- ignoring energy recapture -- is an environmentally beneficial action, which is why landfills sometimes have flares.
But you have to grow that biomass first. After you did that, you can let is decompose (bad) or burn it (less bad), but you could just skip growing it entirely.
True, though quickly is still a period of time that is detrimental to the planet. Well obviously it was self-balancing historically, but add the natural methane with the enormous CO2 of humanity and you have a problem.
Putting long-accumulated carbon back into the atmosphere does not look green to me (unless reaching the very-high levels of CO2 that were present several hundreds of millions of years ago can be defined as 'green')
Biomass is green because it does not change the percent of CO2 in the atmosphere: it fixes it into wood first, to put it back later. The net effect is zero over the human timescale.
Burning coal has also a net effect of zero, but over the Earth lifespan timescale, which is not very good for the current ecosystem (including us).
Biofuels is also destroying ecosystems because of the use of pesticides and fertilizers you a sharp decrease of the number of insects living in fields, decreasing the number of birds. In Germany, in 30 years, we had a 90% decrease of the number of "field birds". This is really worrying but at least, it is now a known fact and the EU agricultural subventions are going to shift to help prevent such issues. Still a long way to go...
Of course, existing coal plants are dirty, but for that kind of money you could make like really nice coal plants.