Once you read the opinion, the due process issue becomes very apparent.
In expedited removals, the accused is not entitled to counsel, they don't appear before a judge, etc. It's truly expedited as the entire process can be finished within a few days. Which is fine, provided there are guardrails to ensure that the people subjected to the expedited process are likely to be illegal immigrants.
The guardrail in question here is that the illegal immigrant must be encountered within 100 miles of the land border. This makes sense because they're close enough to the border that the government is less likely to pick up the wrong people. However, once you remove the 100 mile barrier, it becomes a nationwide scenario for non-citizens. And non-citizens isn't just illegal immigrants. It includes people on work visas, student visas, etc. If people on valid visas are erroneously picked up far from the border where there they don't expect to run into this expedited process and then removed without a hearing or a chance to call a lawyer, get their paperwork, etc., there's a problem.