Free-running snares aren't illegal...'owever they should not be set in places where non-target animals could be caught. Legally they have to be checked daily...preferably soon after dawn then again before dusk to prevent prolonged distress of the held animal...it's the same with any type of trap really.
Seems like the snares you've come across could be illegally set?
If in doubt you need to report such incidences to your local Police Wildlife Liaison Officer.