The fact that leave is unavailable only screens the theft which occurs once you do get leave.
You can see concrete examples of this in a number of the preceding posts. If you are sufficiently unfortunate to take a small number of leave days in a calendar month it is quite easy to see that in many cases you will be assigned the same number of hours as you would have flown in a full month of work. Worse, take your roster which has say 7 days leave. Add to those 7 days the number of XX days you have in the remainder of the month. That total number of days free of work you have attained with this tally will often be LESS than pilots who took no leave at all - you will have expended your vacation days and yet had less time off than you would have had you NOT taken holidays.
Cynical, high-handed, morally bereft theft of your leave. Full stop.