This is OK for small hubs with low power devices plugged into it. A good example is this is a keyboard with an powered hub built-in. This has plenty of power for plugging in a mouse or track pad. Powered hubs have their own power supplies and can supply full power to all the devices that can physically plugged into them. Of course, you do have an extra wire and power block plugged into the wall.
Some hubs can operate either way.
When you plug-in their power supplied, they function as powered hubs. Low speed mode is often used by slower devices such as mice, keyboards, and joysticks. High speed mode is required by faster devices like scanners and printers. Today this speed range places USB firmly in the midrange. SCSI and Firewire are faster, serial and ADB are slower.