I also run a home daycare and I am part of an association of women who also run home daycares. It is very common to charge for the "spot" vs. the # of days a child is in care. Each daycare is a private business and the provider can create her policies however she desires within the confines of the law. If you decide to enroll your child and sign off on the policies, you are agreeing to her policies as written.
I have had my son in child care before and I currently provide care for other people, so I can see this issue from both sides of the fence. As a parent, it is difficult to pay for "days you don't need"...but you also want to balance that with quality care for your child. As a parent, if I need to pay the same amount per week regardless in order secure a safe place for my child to be at a high-quality child care, I will. In my eyes, when my child was in care, his provider had THE MOST IMPORTANT job in the world! She was taking care of the one person that meant the most to me. If you don't trust your provider or are not satisfied with the job she is doing, by all means find someone else..but if you are happy with her, it is probably worth the peace of mind.
As a provider, it is even more important to me that I be able to have a reliable income BECAUSE I only have 6 children in my daycare. I, like many providers, need the additional income the daycare provides. I ensure a reliable income for my family by filling a certain number of spots. If I did not get paid the same amount each month, I would not be able to count on being able to pay my bills (including those associated with providing care for the other children) and I would not be able to continue in this business. I am sure it is different for each child care provider, but approximately 2/3 of the gross income I bring in each month goes back into the daycare - for food, activities, preschool curriculum, toys, additional utilities used because we are open 10 hrs/day, etc...
Child care providers do one of the hardest and most important jobs, in my opinion. They care for our most precious family members. They work long, hard hours. They often do their work alone - teaching, cooking, cleaning, diapering, guiding, playing, etc... And they do all of this for relatively little pay - if you think about the amount they actually get to keep (after expenses) and the amount they are getting per child. I was shocked to find out that the average teenage babysitter gets paid about 3x or more per child than I do for full-time child care... and that is BEFORE I pay for expenses to run the daycare in the first place.
I am an elementary teacher who decided to stay home with my son and open a daycare. This was my choice and I do not regret it...despite the long hours, hard work, and relatively low pay. I have many intangible rewards each day and each week. I feel I understand both the parent and provider perspectives and I think that helps in my business. I work with my parents as much as possible, but also understand that I have to provide for my family as well. If it is not a mutually beneficial relationship for both parties, you should figure something else out.
PS... FYI - I do offer my families a week vacation credit once they have been enrolled for over a year. I take two weeks vacation each year. I have certain paid holidays each year. Additional days I need to close the daycare - due to illness or for personal reasons - are unpaid. I always let my parents know about a closure ASAP. (Two months or more ahead of time for planned days...and as soon as I know about a serious illness that will affect my hours.)