Clone manufacturers make these products and sell them in the market. Most of the times, they get the same die as the original manufacturer, but the quality of testing would be very poor. (If they do any post production test at all). Clone makers would not stamp the original company logo. These may be OK to but compared to fake or defective ICs being sold online as described below.
In other cases, there are people who buy the parts which are rejected by Altera (or other companies) in production. These parts may have some IO failure, high current consumption or some functional errors. They just sell them to un-suspecting buyers. They erase the original company's logo in order to avoid any troubles. These parts will screw the buyer, as they will most likely not function as expected.