Step Up can limit ONLY based on what's in the statute. They cannot disqualify any curriculum that fits the criteria for "curriculum" under statute. There are some limitations for ONLINE programs, but that's because that's in statute, too.
Legislation gave Step Up authority to provide a platform where companies who provide curriculum for home education families can allow families to purchase curriculum directly without having to pay up front and be reimbursed. (That platform has yet to be released as of the date of this post, but it's coming.)
However, the curricula on that platform are not the ONLY curricula a parent can use--they are just the ones who agreed to participate in the direct-pay arrangement.
If you want to purchase a different curriculum not available on that platform, you would just need to buy it up front and then wait to be reimbursed.
Step Up's system (EMA) also has a place where you can request preapproval, so you'll know for sure you will be reimbursed for a particular purchase. If enough people request preapproval for a particular curriculum, Step Up will try to work with that curriculum and get it on the direct-pay platform.
Step Up really is working to make this as beneficial to parents as possible, so if there's something you want to see added to the direct-pay platform or the purchasing guide categories, be sure to reach out to Step Up and give them that feedback!