First of all, to summarize what you're saying: you're taking a lot of time off work for doctors appointments, and you think the company should be compensating you by paying you for this time you are not at work. I won't speak to that perspective, but I can speak to the law, and our company policy as written and unwritten.
I am an agent of the company and administrate our HR policy as I see fit for my employees. It sounds like you're taking an adversarial approach which will get you nowhere. Having a boss who's willing to go to bat for you (or ignore some of your time off) is going to get you a lot further than complaining to HR (who, I'm sure, has made sure all their company policies are legal--labor laws are fairly simple). You may also want to consider that "not paying you for time you're not working" is perfectly legal.
First off, in California, there is a classification of employee (assigned by the state) that is determined by >job function<, NOT by years worked at a company, importance to management, or whatever. These are "exempt" (what you call salaried) and "non-exempt". Exempt employees have more flexibility in their hours, but they also can be legally required to work 100 hour weeks. Non-exempt employees (hourly) have little flexibility in their hours, but also legally have access to benefits like overtime if they are required to work hours greater than a certain amount in a given period (more than an 8 hour shift, or more than 40 hours in a week, for example).
That frames this whole issue as a variable, since you have not told me your job function. A company can not simply see that an employee is pulling down mega-overtime and reclassify them as exempt (salaried) to save themselves some dough. There is no choice in the matter in this state.
With non-exempt classified employees (classification is determined by the state based on your job function), a managers discression on flexing time over several days is limited due to overtime laws. For example, allowing you to work 11 hours on a day following a day you worked 5 is a little sketchy if you're "non-exempt". This is the question I would ask HR: "is my job function classified as exempt or non-exempt?" You may be getting the same check each month (making you think you're "salaried"), but that doesn't really mean anything if HR isn't on the ball.
Anyway... OUR official company policy for both exempt and non-exempt employees is that an employee gets 40 hours of Personal Time Off/Sick Time (PTO) each calendar year. If an employee does not take it in a calendar year, it goes AWAY (this is fairly normal and perfectly legal in california). That is, PTO/sick time does NOT accrue.
Employees are allowed to use PTO in any way they see fit, and managers are expected to approve this time with little notice by the employee and little prejudice. This allows employees to have some time for handling personal business for situations like yours or for handling that expired reg on their vehicle they forgot about or whatever.
Our vacation time is the time employees get for rest and it is at the discression of managers when AND if employees can take it or not (this is a small company, and depending on what's going on, an inconvienient vacation could affect business performance in a big way). To make up for this less discressionary benefit, vacation accrues forever.
Since the only thing vacation requires is supervisor approval, if one of my employees knows about some appointments several days in advance I have often approved the use of vacation to cover as well (I had an employee who got braces last year, and it was a similar situtation--she ran out of PTO time).
That said, the "unwritten policy" used at our company by most managers is that if an employee has some time they need to be away from work, it's perfectly acceptable to work with that employee and let them "flex" their schedule to compensate. Most managers here require the time to be less than a couple hours, and it must be made up within 2 days of the event, but I know of some managers who've let this go as far as a couple weeks for exempt employees.
Personally, I've even allowed some employees to sort of "wash the time out" if there's some weekend work coming up in the next couple weeks. Since this requires cooperation from your manager, you really need to get their help with this type of set-up.
That is, if your manager goes strictly by the rules, they do not have to pay you for time you have not worked. Make a friend who sympathizes with your situation, and you'd be surprised at how much you can get away with (especially considering if you're a superstar employee).
Hang in there with the health thing. I went through a total of 6 weeks of chemotherapy over 6 months and I wasn't docked a single hour of any kind of time. Fit as a fiddle, these days.