Photography at public event

[pearson]

I spent last Saturday at a mountain bike endurance event taking a few photos. The event was held on public land in a reserve often used for mountain biking/walking, etc. I have taken photos of many competitors of whom my friends were racing amongst. I have posted images of my friends on my FB page among some photos of other racers.
This afternoon I received an email from the club president stating that it had come to his attention that a number of professional photographers including myself had taken photos of the event without permission of the club and that each photographer would be contacted to ascertain the intended use for the photos and to quote "Please note that ***** Mountain Bikers has not given you permission to utilise these event photos for commercial use".
At no time was there signage notifying anyone that the public land on which this event was held was closed to the public. There was no signage stating photographs could not be taken. My intended use of the images in naturally to sell as many as I can to interested parties. I was going to supply watermarked images to the club for their webpage free of charge as a one off...maybe not now however.

Can anyone tell me where I stand in this? I would have thought because it was held in a public place they have no real argument but I'm just not sure...

Thanks in advance!