A number of factors have contributed to the "electrostats don't have bass" myth:
1. Impact vs SPL. Saying they don't have bass is implying the frequency response of the bass is shelved down or rolled off. Properly sealed electrostats are flat in the bass, but most electrostats do have less bass impact than your typical dynamic. Some of this is due to the limited excursion of the diaphragm. The distance between the diaphragm and stators has to be kept really small though so the electrostatic force can act upon them. You could increase the spacer thickness, but you'd need to greatly increase the bias voltage.
2. Reputation of older Stax headphones. The really old Stax headphones had smaller drivers and kinda crappy earpads that didn't seal well, resulting in bass roll off. Even many of the Lambdas can be hard to get a perfect seal with. When you break the seal of an electrostat, you start getting roll off below the resonance frequency which happens to be in the bass spectrum. Once you are able to get a perfect seal, though, the frequency response will be flat beyond the audible range of human hearing (around 20 Hz).
3. Reputation of electrostatic speakers. Speakers are naturally open air and not sealed to your ears, so all electrostatic speakers roll off beyond the resonance frequency. In order to "push" this frequency down (as in roll off less/lower in the bass), you need to use larger panels. This quickly gets out of hand though, as the panels required to do flat sub bass would be enormous. Audiophiles seem to have a tendency to apply laws that work for certain components to other components as well, even when it makes no sense ( electrostatic speakers -> headphones, turntable anti vibration -> everything needs vibration control, turntables needing to be set at a perfect RPM leading to the birth of "PRAT" -> everything has PRAT, early digital was crap -> digital will always be crap, etc. ).
4. Typical bass response of electrostats vs dynamics. As mentioned earlier, a well sealed electrostat or planar magnetic headphone will be completely flat in the bass and sub bass. A typical (good) dynamic's frequency response looks a little different. They tend to have a little hump in the mid bass (slight bass emphasis) and then roll off a little in the sub bass (slight bass deficiency). You can see why many of us say Stax have an accurate level of bass. I think many audiophiles are accustomed to that slight emphasis in the mid bass.