I think the first person had intentionally not tried to get away with the two pack of nails without paying it.
If he often bought things from the shop, the shop owner or th guard will likely know well of him. Even if he really forgets to pay one or two items, he won't be in trouble. No need to call police.
The second lady is obviously caught in the video. She can ask the store to replay the video to prove she is innocent. Apparently she has something to hide.
Otherwise she can also tell the police the guard is trying to frame her.