Protest movements require money - to print pamphlets, make banners, buy candles for candle-light vigels etc - and those who came to the event were encouraged to give ‘donations’ of 5 pounds or more to help with fund-raising. Though one of the organisers did say that students could be exempt from the entrance price.

“That night you broke us.
You broke everything.
You thieved.
You stole.
You usurped.
You kidnapped.
You thieved, you stole, you usurped,
You kidnapped us from us.
You maimed.
You sabotaged.
You raped.
You looted.
You punished.
You shot.
You whipped.
You hanged.
You executed.
You killed.
And all this time while you did all this, you managed to put on your white robes and make pilgrimages for your eternal salvation. And you made us pay.
Pay for your prayers.
Pay for your piety.
Pay for your zeal.
In my house, that night, when all was locked up, safe, concealed and the lamplight glowed upon my face and those of my sleeping family, when all was well, that night you broke in. You broke me. You dragged me out.
You punched me.
You slapped me.
Your crushed me.
You smashed me.
You told me this was your house. Your house. Not mine. Not ours. Your house. Yours, yours, yours to do whatever you wanted to. Yours to decide to keep or destroy. Just yours, yours, yours!
…..We were not to do anything that you did not think we should. We were paralyzed, we were confined, you had imposed your law upon us….”

From the novel “Stay With Me” by Maniza Naqvi

Though it is not my function but I have seen the full advertisement. It has some prominent writers who read from their published and yet to be published novels. I suppose authors get paid for their time, especially in a busy city like London.

Still you have a valid point and I think they should have got some sponsorship (but who sponsors these events that challenge regimes!!)