I do understand the part where some organisations opt for one for one exchange if they would not have enough to give around, but I fail to understand why do some states mandate it?

What are they trying to achieve, if anything the discarded syringes are more likely to actually be infectious since they will probably be reused/shared if given out as a part of one for one exchanges.

And last but not least, how seriously are one for one requirements taken, especially in the states that require it, is the amount generally only (generously) estimated or are the syringes brought back counted one by one?