For the most part they are just scams. It is a perfect sell for scam artists to use. When people hear this, they tend to think of it as compiling a bunch of reports and entering it into some spreadsheet or database; basically what you'd expect a secretary at some company to do. And if it's all on the computer, than it seems plausible that a company would be able to do these tasks on-line and so can hire anyone anywhere to do their data-entry. However, this is usually not what the actual "data-entry" jobs you see posted on-line are all about.
"Data-entry" can mean any data that gets entered into some worksheet or form. So from that, some "data-entry" positions are nothing more than advertising jobs. You see, as I am typing in this post, I am actually entering in data; I had to enter in a subject, choose an icon, and type in a message (and sign-on with a username and password). So some "data-entry" jobs will just have people sign-up for some forum site (like FC), spam a bunch of advertisements and / or links, and collect a percentage of the advertising money for each person that clicks thru a link or purchases something using a unique offer code in the advertisement. The reason why the company cannot to this themselves is that most forums have restrictions on this type of spamming, and will drop the user in a heartbeat. It's just easier for the company to get a bunch of other people to sacrifice their accounts to do their spamming for them.
Other "data-entry" jobs are really nothing more than GPT sites like FC. For example, if you go to the Offers section, you can get paid to sign up for some survey site. When you sign up for that survey site, you usually have to fill out a form (name, e-mail, age, sex, income bracket, etc.) so that the survey site can use this data to screen for various demographics. When you filled the sign-up form, you are doing data-entry. And after you sign up for the site, the GPT site will pay you some money for signing up. Hence you got paid for "data-entry".
There are some more legit "data-entry" jobs, but this sometimes takes more skill than the average work-at-home seeker... There are a few places for medical and legal transcriptionists, for example, but these jobs usually take advanced training. Also, there are the human-task oriented sites (like the tasks section in Various Offers of FC, or Amazon's MTurks) that require a human to give or verify different data that a machine itself wouldn't be able to do (eg select pictures that pertain to a particular topic, determine if the address data for a particular company is correct based off of the company's web-site, etc.) Again, it is "data-entry" in a broad sense of the term, but because they usually don't take too much skill, the pay is rather low.