It's a balance between encouraging participation and preventing abuse. Removing the $100 spend limit essentially requires the company to then monitor and blacklist as needed. Enabling a whitelist approach as suggested is likely safer, but it does require referrers to do some work to apply for whitelist or the company to do work to acquire referrers.
Frankly I'm fine with either one - I'd go with whatever is lower effort for the company.
RE: Splinterlands: Expand Referral Program