It's probably because full timers don't get their bank holiday rolled up into their holiday entitlement, so, they get 22 days to take, plus they get all the bank holidays off.
You're entitled to the same amount of time, pro rata'd. For part timers it is easier to roll it up into the overall holiday entitlement, and then get you to take holiday on a bank holiday you would normally be working on, because....
You are only getting 60% of a bank holiday, so, if you didn't roll them up, on a bank holiday, in order to take the full day off (and not just the 60%) you are entitled to, you would also have to take 40% of a day's holiday to cover it.
On the other hand, if you are not working on bank holidays, you would never get the benefit, but by rolling it in with your leave entitlement, you are guaranteed the same amount of time off.
Oh, I realise now how hard it is to explain.
I manage holiday for part-timers where I am and we do exactly this. It is the fairest way for you - and, as MrsJamesMartin says, tends to work in your favour, as you get to take the time you have earned for bank holidays when you wouldn't have been working anyway, on days when you would.