Lured into a physical shop by a voucher they are likely to by things they would have bought elsewhere, the effort to visit another shop not being justified.
A similar effect happens online with free shipping over a certain dollar amount, which incentivizes getting as many things as you can from a single site.
Alternatively, free shipping (or free shipping at quite a low threshold for what's being sold) is sufficiently normalized that if shipping is not free, some percentage of users will just close the window and move on.
