I reckon these vouchers work just fine - it is not a 'con'.
If you think about it, the cost of running this sort of attraction is pretty-much fixed costs, so anything the owners can do to get more punters through the door during the low-season is OK for them. The way they choose to do this is to create pricing differentiation by these 2-for-1 offers for those punters on a tighter budget who are willing to go to a bit of trouble to search out the deals. It creates a demand when such demand would not exist at all if they didn't do it.