The fact is when the Biharis come back, the people who have been placed by Sena must vacate their positions to make way for those Biharis.
The reason the Biharis quit Maharashtra was not due to their own will but due to fear and pressure from Raj 'The Stupid' Thackrey. So they have a very strong case if they go to a labour court to contest to get their jobs back. Also employers would rather have someone back who knows the job than someone who has just learnt it a few weeks back and is still learning.
Basically Sena is taking the marathi workers for a ride by promising them with jobs that are not sustainable.