Some employers may be unfamiliar with the rules for employing international graduates and may even think that it is not possible.
The Association of Graduate Careers Advisory Services (AGCAS) has produced a leaflet to help employers understand the rules and regulations for employing international graduates. You can download a copy of the leaflet from the AGCAS website.
An employer may be acting illegally if they automatically reject your job application because you are not settled in the UK or because you require permission to work in the UK. If they could sponsor you under Tier 2, then they may be committing unlawful discrimination under the Equality Act 2010. The UK Government has published a document to help employers understand their responsibilities: Avoiding discrimination while preventing illegal working: code of practice.
If you are concerned that an employer's recruitment process may be discriminating against you unlawfully (e.g. automatically rejecting your application), you could contact your potential employer and use the above resources to explain the situation.