I suppose I’m a few months late to this discussion.
However, allow me to state that the objections raised by Nop are valid.
A simple Google search will lead you to an analysis of the the bill put forth by Senator Sanders, which shows that, yes, the bill would apply to part-time employees.
The text of the bill itself (PDF), at the bottom of page 4, confirms this.
The objections you raise are ones that Sanders and his staffers, no doubt, were well aware of — which is why they sought to include part-time employees.
I do understand why one might assume that the bill doesn’t apply to part-time employees: it seems unreasonable to expect an employer to carry the burden of an employee receiving federal benefits, if that employee is the one choosing to only work 15 hours a week.
But realistically, there are very few people choosing to work as little as that. Most people who work part-time do so because their employers will not hire them on full-time.
The fair thing would be to include provisions in the bill which grant businesses exemption from the tax only in cases where part-time employees have no other part-time jobs.
On the other hand, perhaps Sanders realizes the bill is being unfair in such cases, but doesn’t mind being unfair to employers who purposefully hire only part-time employers so they don’t have to pay them full benefits. Perhaps the bill is intentionally being unfair in one area, in order to encourage employers to hire more full-time employees.
Now, with all of that said, I do agree with your assessment: I do not for a minute think that this bill is responsible for Amazon’s wage increases. Corporations do not generally make major decisions like this in reaction to legislation that has not even passed yet.