Retailers are quite used to running 15-20% discount sales. Many also offer round the year 5% discount on pre-paid cards sold to club members (automobile club, employee club members etc). Paying 4% margin to APT sits well in comparison.
Ticketing agencies and event managers aren't as yet accustomed to offering such per-sale discounts. At best they may offer bulk discounts.
Not to say it will not happen if they can see volume growth from linking with Afterpay.