All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
According to the Fiqh rules, anything whose owner discards because he no longer wants it, others are allowed to take and benefit from. Abu Al-Barakaat ibn Taymiyyah (the grandfather) wrote, "What people discard for not wanting it anymore becomes the property of whoever takes it." [Al-Muharrar]
As to whether these receipts were left intentionally or forgetfully by their owners, this cannot be established or negated by scholars. Rather, people who have experience in this regard can judge this, and circumstantial evidence and what people commonly do is to be looked at.
So if these receipts and the resulting earned points are not eagerly sought after by average people, then they are considered property whose ownership is legitimately obtained by taking it without having to announce it, even if the original owner dropped it unintentionally.
Another question remains: Does the store allow a person to benefit from the receipt of someone else? If the store does not allow it, then it is impermissible to benefit from it. The Prophet, sallallahu ‘alayhi wa sallam, said, "The Muslims are bound by their conditions, except a condition that makes lawful what is unlawful or makes unlawful what is lawful." [At-Tirmithi: Hasan Saheeh (good or sound)]
The fact that the cashier gives someone the receipts of others does not necessarily mean that those receipts were left intentionally; however, it does indicate that the store does not forbid that someone benefits of the receipt of another.
Allah knows best.