It's a standard contract of employment, and you'll be at the airfield (irrespective of weather) from 8am to sunset, 5 days a week to include Saturday and Sunday. So even if it wasn't forbidden in the contract, you'd probably struggle to find time to instruct elsewhere. The £1,000 pcm is a salary of c. £15k, thereby being around the minimum wage (this covers their @rse for demanding you be on site 8am-SS). I believe self-sponsored guys get the £25 for >20 hours, etc., but Cabair sponsored guys don't.
I think I'm correct in saying they're the only outfit to offer sponsorship; I think they deserve some credit for getting people into paid flying jobs that might previously have not been, even if they do drape you over a barrel... Unless you've got a wife and kids, Cabair's inferior offer has got to be better than nothing, and another 2 years in a job you gave up because you wanted to fly.