If, in the case of outfits like Redbeacon, the site is collecting leads and distributing them while letting the two parties work out the arrangement for themselves, the site is a referrer and the contractor is a contractor. Redbeacon doesn't require that the contractor who I hire to paint a room show up in Redbeacon attire, only do work through Redbeacon, charge what Redbeacon dictates, and follow Redbeacon's rules for how the work is done.
In the former case, people don't say "I hired J. Q. Driver to take me from my office to the airport," they say "I got an Uber." It's not "Local Delivery Service, LLC delivered my groceries today," it's "Instacart brought the milk." Being a part-time or irregularly-scheduled employee doesn't make someone any less of an employee than a 40-hours-per-week office worker.
The problem is one of enforcement. Just like how Uber got its nose under the tent by bullying through existing rules, companies like these exploit the fact that state Departments of Labor are politically hesitant (can't be crushing those small businesses, after all) and the federal DoL is limited.