There is nothing exceptional about what this child's mother is doing. My children spent occasional nights with their grandparents from birth, but never stayed for more than one night until they were 7 or 8. It was much the same with my grandchildren, they live a distance away, but didn't spend even one night alone with us until they were at least 6. Our grandson, would not sleep anywhere without a parent, not even sleepovers with friends until he was 9.
Looking after a child a couple of days a week, is very different from taking them away on holiday for several days and you say nothing about your age or general health and whether there will be a second adult with you. An older person away with a young child could be taken ill, or have some event take place they have difficulty dealing with. If there is a second adult present, a partner or friend, rather than a blood grandfather, the parents may have concerns, even if these are baseless. I think you are really being rather unreasonable to assume that because you are not allowed to take a 2 year old away for a few days that the day care you give your grandchild is exploititive.
You also write of 'son x girlfriend'. I deduce that they live together. Most people would speak of son and partner, using girlfriend if he is in a long-term relationship sounds very grudging, as if you resent the partnership.
Grandparents, especially grandmothers, who care for grandchildren, need to constantly remind themselves that they are not the child's parents, and they need to leave a proper emotional distance between being a parent or grandparent.
Children and grandparents, can be suddenly separated from each other for all kinds of reasons, including moving away for work, grandparents suddenly being too ill to care.