Your diagram illustrates a standing wave, rather than simple wave-propagation on a long transmission line. The waves sent out by the 500KHz signal generator are being reflected by the infinite-Z located at the end of the unterminated line. The waveforms are dynamically changing over time (and hence the dotted lines in the diagram.) To get rid of this effect, either use an infinitely-long line, or add a 75-ohm terminating resistor.
Instead, if assuming an ideal infinite transmission line, the current and voltage are at zero phase, not 90deg, and the line behaves as a resistor (an energy-storage medium having characteristic impedance: volts times amps gives single-direction watts.) Fifty-ohm and 75-ohm lines have 0deg between their V and I waveforms, same as with 300-ohm twinlead, or that 'magical' 377 ohms of waves in empty space.
If the VI phase wasn't zero, this would produce some negative watts during part of the cycle, which is the same as having some wave-propagation in the backwards direction.
Note that something here is orthogonal: the e-field and b-field vectors of the flux surrounding the conductors is everywhere at 90deg. Simplified version: each conductor is surrounded by circular b-field, and each conductor is surrounded by radial e-field, where the flux-lines cross each other at 90deg. In non-tech explanations, this is often described as "EM waves have transverse E and M at 90deg." But it's not the waves that are 90deg, instead it's the flux-lines. Therefore, it applies to circuitry at DC, not just applying to waveguides and light waves. (Even in a flashlight, the propagating EM circuit-energy includes orthogonal flux lines, E-cross-M giving power density of unidirectional energy flow.)