Is listener's fees in a conference justified? In a conference, where I am an author, I noticed that there is a substantial fees for registering as a listener for the conference.(10-30% less than the fees for professionals, students etc  but it is still on higher side).

Is this justified given that as an author I would like my work to be presented in front of as many people as possible and receive comments for the same. Is this common practice in conferences across Computer Science or any other field ?
The main reason for fees is that there is a cost for renting the auditoriums and perhaps other services (refreshments, facilities etc.). Under such circumstances all participants need to pay to support the conference. Exactly how the pricing is done differs widely but in most cases it is based on costs divided by a certain number of projected participants in different categories. There are conferences which I have attended that seem horrendously expensive and are held in luxurious places with organizing services and then there are those run by a smaller society where making a cheap efficient meeting for the members by the members is a priority. In all cases you pay just to get in.

So pricing will vary depending on venue and organiser. Depending on how the conference is organized you may be able to gain some insights into how the costs come about and then make constructive comments to the organisers on how it should change; particularly if the organisers are a membership organisation where you are a member.