That Wall Warts Dilemma
The answer to your last question is that you can use them on the electronics they are designed to work with, since the designer took into account the unregulated nature of that kind of AC adapter (quickly becoming an endangered species, by the way, as regulated types take over).
An unregulated adapter is typically nothing more than a transformer, a bridge rectifier and an electrolytic capacitor. The average voltage drops as ripple increases, plus the transformer has some output impedance and the diodes drop more voltage as current goes up. So the average output voltage decreases and output ripple increases the higher the load current is. Typically the output voltage is specified at the maximum load.
The more common type nowadays is the regulated switching adapter which are typically much smaller and lighter for the same amount of output power. They do drop in voltage slightly from no-load to full-load, but it's typically a small amount (like maybe &lt;2%).