Because you cannot deceive human behaviour... the moment you think you are tricking a person, you've already lost. Help your users moderate their time online and you will keep them coming back. If they become addicted to your site, then you will lose them completely... near guaranteed. People have partners, children, employment, friends, etc... they are far more over-powering forces than an admin trying to keep them on their website. By helping users manage their time you show them a level of trust, a degree of respect for their time. Psychology dictates the best things in life are those done in moderation. Moderation often helps a person to return... When you move into addictive behaviours, the person will at some point completely shift to the opposite end of the spectrum, being complete avoidance, hence you lose them.
Suggestions and tools such as this will only help those who chose to recognise and moderate their online time. Some will use it as a tool to actually compete for the longest time online, so as an admin you win by giving them the very tool to assist them to moderate their online time. There are usually more positives to such a feature than negatives when measured with human behavioural science.