The red rubber bands are indeed down to postmen. Royal Mail rubber bands come in two sizes, erm - large and small - and they cost Royal Mail millions every year. For one thing a lot of them break, especially the small ones, because they are frequently overstretched. They are only meant to go round a bundle of letters side-on, not lengthways (that's what the large bands are for), but you still see postmen trying to stretch the small ones to the length of a bundle of A5 or A4 correspondence because they are too lazy to search the depot for a handful of large size, which are like the proverbial gold dust. Then the small bands they used incorrectly snap anyway.
When I was delivering I used to finish the day with a satchel pocket absolutely stuffed with rubber bands for re-use the next day. It makes sense to keep them because you don't want to waste time in the morning searching around for replacements, especially the large size. However, red rubber bands get dropped all over the place because some postman are too lazy, careless or wasteful to pocket them for re-use, but the postmen aren't exclusively to blame: some of the delivery pouches are so worn out they have holes in, so you may scrupulously deposit every used rubber band in your satchel only to find at some point along your route that you've been losing a lot of them out of a hole in the compartment you put them in.
Re: string. We sell string in the supermarket where I work. It doesn't have a very great breaking strain. I know because I used some to tie a bundle of newspapers weighing no more than 10 kilos, and the string broke when I picked it up. Thenceforth I resolved to use garden twine instead or double up on lengths of string for greater resistance.