Why not? Because every pilot who works for free is worsening the job market fot everyone including themselves.
If you're qualified to fly the aircraft, then you should be paid. If you're not, then what sort of operation are they running?
Working for free (apart from busting minimum wage legislation) means you would have no employment contract, no employment rights, and would generally get treated like a doormat.
At what point would this charter company think, "Let's pay this guy a wage"? They won't .... they will continue to use you until you move on and then they will use someone else. The trouble is that people do not value what they get for free and you would not be a valued member of staff; you (along with everyone else who does the same thing) will be treated as expendable.
On the whole, you would lose the respect of other working pilots who are managing to earn a small wage. These are the networking contacts you need but you need them onside. By working for free, businesses will come to expect it and the more pilots who work for free will encourage other businesses to cut wages.
Cheers
Whirls