They don’t give a reason for this, but I assume it has to do with taxes. You see, if you pay someone $600 in any given year, you are required by law to report that earning and send them a tax form at the end of the year. And since 60,000 equals to $600, I assume they have that rule in place so they don’t have to deal with all that paperwork come tax time.
You're unlikely to "get rich quick" by taking paid online surveys. You will, however, likely earn or win some extra spending money, or free or discounted goods or services. Doing paid online surveys can be seen as a way to get a fairly steady flow of a decent amount of cash coming in each month. If you enjoy participating in online surveys (especially if you enjoy sharing your opinion for prizes, coupons, and other more typical non-monetary earnings), then paid online surveys is a good choice for you.
Paid survey site companies invest billions of dollars into marketing research online. They always want to find out more intricate details of a consumer's relationship with their product. Whether it's a new television show or just an existing product line, companies like Disney, Samsung, and Sony go to great lengths to find out their ideal audiences.