This must be complete rubbish, and I would hazard to say that it'd be a small handful of schools that'd make such claims.
No school in California that I have conversed with, which includes pretty much every single one of them, has had anything at all negative to say about summer courses!!
Remember though, sample size/type bias! So, to be truly prepared, you need to consider what schools you'd like to apply to, and call/email them directly about the issue, and then it'll be resolved!
The response from Passion4Sci is accurate; however, let me add another level of clarification.
Regionally accredited (the most stringent type of accred available to colleges and universities) institutions agree to accept courses from other regionally accredited institutions as part of their agreement to stand for review. The length of term (and, increasingly, the format) in which the course occurred is a non-issue. This is one part of the overall quality-control process that accreditation supports.
As such, as long as a course is a formal offering (taught by qualified faculty) at one institution, another institution should accept it; in states with formal written articulation agreements between schools (most usually between community colleges and the state university systems schools) these courses move without any hiccup because articulation agreements are binding agreements.
The only exceptions are for course that do not fit within a particular school's curricular requirements, for example, if a Montana state univ required all students to take Montana history and literature, it would not have to accept American history/literature as a replacement (the school's Registrar would take this course as an elective course, though, and, thus, still grant the legitimacy of the course itself, even if it didn't meet pre-req or program requirements).
There is little-to-no mystery in the area of course transfer because of the legal underpinnings that tie institutions via articulation agreements and via accreditation processes. Schools do have the right, however, to reject courses taken an unaccredited colleges, because these institutions do not participate in the peer-review process that helps to engender trust about quality of educational experiences.
There's a lot of false information that circulates in any community. Just as there are urban myths about poodles put in microwaves and legions of gators in the NYC sewer system, so too do academic rumors swirl. The best answer always is to read the Catalog and talk to the admissions personnel at the institutions you are interested in. These personnel know their rules and regulations and are charged with sharing those with all interested parties.
Hope this more-detailed answer is what you needed to better understand the issue.
Eric H. Hobson, Ph.D.
Associate Dean
Belmont USOP
