What if a faculty member is not successful in obtaining the summer salary from a funding agency? I am a postdoc in Physics areas and trying to get myself ready for applying for tenure track positions. However, the US academic salary system seems still mysterious to me.
In several salary surveys, a reasonably close figure for starting salaries of starting assistant professor positions emerge as $75-80K per year. The point is that, as far as I understand it, the salary is based on the calender year basis. However, the contracts for the assistant professors are usually academic year basis (except that probably in the first couple of years they may get this full salary in their start up package). 
This means that practically they get $55-60K for 8-9 months. The rest of the salary, called the summer salary, needs to be obtained via applying to the respective funding agencies.

However, funding is obviously not guaranteed every year. So it is obviously out of question that people should be based on the full year salary amounts shown above. Truthfully, their salaries are $55-60k per calender year and if they are able to get some funding, they can get some bonus over the summer.

Now, the first question is: isn't it way too low of a starting salary for a PhD in STEM areas?! Especially, people are normally already in their mid 30s when they get a tenure track position so they usually have a family to support. Sure, there are many people survive with much lower salaries and also the stereotypical argument that you don't enter academia if you want to earn money is also around. But I am talking about skills, experience and qualification. Anyway, it just seems too low salary to me and I would really want to know if there is something crucial I am missing here.

Secondly, what do faculty members do if they don't get the summer salaries, what other options are there for them to compensate the money.
In the US, salaries for tenure-track positions are usually for the academic year; one might get the summer salary thrown in as part of the startup package for the first year or two.  So your academic-year (9-month) salary of $70K would translate to a full-year salary of $70*12/9 = around $93K.  The expectation would be that you would use those first two years to establish a research program, write grant proposals, etc., so as to be able to fend for yourself after that.

While it's true that grants are getting harder to get, remember that single-investigator grants are not your only option: you can also participate in larger multi-PI and collaborative proposals.  For example, you may be able to put together a collaborative proposal with your dissertation advisor while at the same time striking out on your own -- his greater seniority and track record may make it easier to land such a grant.

Finally, I would counsel against spending a lot of time teaching during the summer --- IMHO you'd be better off spending the time moving your research forward (remember why your initial summer salary was included in your startup package).  One reason (not the only one) is that having a solid record of peer-reviewed external funding greatly strengthens your case when you go up for tenure.