For a hiring manager at a large company (10000+ employees), that interviewer probably won't mentally compute the healthcare costs of a candidate. Although even large companies like AOL were not immune to high medical costs -- even if they inadvertently presented it in a distasteful way.[1]

However, for small companies (e.g. 10 employees), the premiums to fund the health insurance pool will change drastically depending on which employees it has. For example, let's say after a candidate interview, the employer (owner) checks Facebook and notices the wife just had a premature baby or birth defect, etc. Let's say that preemie baby's medical costs is $46000 more[2] (or $1 million more in AOL's case[1]) than a normal healthy baby. We as society may want the employer to ignore that costlier number but we should'be be surprised if he'd rather not pay it and hire another cheaper candidate. Yes, it's illegal discrimination, but we have to be realistic about self-interest -- especially when the employer's (real) reason for rejection can't be proven in court.

If you're bringing in big profits like Google Inc, you have the luxury of not having to make distasteful decisions with finite amounts of money in regards to employee healthcare costs.

[1] Slate: http://www.slate.com/articles/double_x/doublex/2014/02/tim_a...

BI: http://www.businessinsider.com/heres-why-tim-armstrong-knew-...

[2] http://health.usnews.com/health-news/family-health/womens-he...

Wouldn't that also be illegal age discrimination for anyone above 40?

In the US employers typically share in the expense of health insurance, so the more dependents an employee has, generally speaking, the more their (non-salary) benefits cost the employer.