People always say similar, but in reality teams are incredibly open to letting drivers go for drives regardless of who it's with. Kvyat was still under contract, though without a drive and Ferrari offered him the test driver role. Ferrari didn't care about his RBR or Renault (at the time) links and offered him the drive and RBR were happy to let him go straight away for that role. Likewise the second they wanted him back for a drive Ferrari were willing to release him immediately.
Teams and drivers work together, but they won't deny a driver an actual drive or a job really at all.
The only reason to stop him going to Haas is if they were very very serious about him getting in the car next year which seems increasingly less likely.