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They are not mining so they don't have any way to confirm it themselves for you. They rely on the free bitcoin transaction accelerator which does the work for them and they just happen to take advantage of it from those who doesn't knows about it.
It was also a problem earlier when there was a high amount of unconfirmed transactions in mempool and IIRC someone reputed came up forward and maintained a list of free bitcoin transaction accelerators so that people knows about it and avoid using any individual to do the acceleration.