The resistance in fact, changes during each sinewave. These devices draw current in pulses. The device then shuts off and current goes to zero. The same thing occurs on the negative side of the sinewave with a second negative pulse of current being drawn. The current drawn then is a series of positive and negative pulses, and not the sinewave drawn by linear systems.
Some systems have different shaped waveforms such as square waves.
These types of systems are often called non-linear systems. The power supplies which draw this type of current are called switched mode power supplies. Once these pulse currents are formed, we have a difficult time analyzing their effect. Power engineers are taught to analyze the effects of sinewaves on power systems. Analyzing the effects of these pulses is much more difficult.