The phone charges more slowly by design when connected to a usb port. By the agreed standard, usb ports are only capable of 0.5 amp. Regular usb chargers have a jumper across the data pins. This tells the phone that it can charge at full rate. When plugged into a usb port, the data pins are not shorted and the phone reduces the charge to below 0.5 anp to prevent damage to the device with the usb port. Those "charge only" cables have no data lines but do have a jumper across the data pins in the micro usb plug. This can bypass the safety feature and damage the device if plugged into a usb port. They should not be sold. There are full function cables with heavy enough power wires to provide maximum charge.
By convention, usb ports are limited to supplying 0.5 amp, so you are correct. The load usually detirmines the charge current and without some mechanism, the phone would fry the usb port. Regular chargers have a jumper across the data lines that tells the phone it is hooked to a charger and can charge at the maximum rate. USB ports are high resistance across the data lines so the phone knows it is plugged into a usb port and reduces the charge rate below 0 .5 amp. to protect the port. This scheme might not be universal but it is common. Samsung uses it.
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