Feet up on hay rack?

[lilyandluna]

I have a midwest cage and I have a small hay rack that looks like this-
View attachment 102618
The only problem is my guinea pigs have to go on there hind legs and lift themselves up to eat hay. It is as low as I can hang it. Is it okay if they go on there hind legs? is it bad for them to do that?

 

[Piggies&buns]

My piggies can access the hay from the rack from anywhere around it, but they still choose to go on their hind legs and put their front feet on the top and eat the hay from the top