Bread, in reasonable quantities does little harm, BUT many waters I know also ban it simply because mindless idiots don't use common sense when using it as a floating bait. Large lumps of crust floating on the surface is an absolute magnet for all forms of water life that spend every day searching for such morsels. It also attracts land gulls which appear out of the blue!
Using bread as a surface bait just needs a bit of thought and it can be perfectly safe if you follow a few simple rules. What you don't do is cast it out and then forget about it which I see from time to time. It needs watching constantly !! Ducks, among others will eventually search it out and if you are not paying attention they will make off with it and the hook !
Personally, I have never seen the need to cast miles out into a lake which attracts ducks almost immediately, but rather fish it close in were ducks etc will not venture. I also fish it under trees close in and in both cases it can be fished perfectly safely...
A more exciting way of fishing is hard to find - seeing a large carp cruise up and suck in a large crust is breath taking - BUT as with all forms of fishing common sense MUST prevail.............
Your sign post saying ' No bread allowed ' sounds like a total ban but I could be wrong. Best you check before fishing..
ps If you are fishing a very small water some clubs will ban bread as many folk, for some reason, find it necessary to load piles of ground bait it over the season and this will do little good for the water. Think about it ; Piles of bread sitting on the bottom with no fish interested - not difficult to work out the water must suffer over time !!
As they say ; USE YOUR LOAF AND THINK BEFORE YOU CAST..............and don't go upsetting other anglers with wayward casting of chunks of bread !!!
Good luck !!