Just to explain why this is the case I will quote MSE:
Stamp duty: When purchasing UK shares expect to pay 0.5% stamp duty and an extra £1 on transactions above £10,000.
This is simply to avoid the £1 tax and, possibly, unnecessary operations required to administer that.
Edit: I understand the query is regarding an ETF (SDRT exempt) but can only assume that it is operationally easier for Freetrade to block everything from executing if it exceeds £10,000