What if a faculty member is not successful in obtaining the summer salary from a funding agency?
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.