Is it frowned upon to wear the field pants with the knee pads removed while in garrison?

You'll be in an infantry battalion, no one will care. But you will look silly if you are wearing them and you don't have any training that day. Also, most of those pants are in the old pattern that is no longer authorized for wear. OEF pattern was phased out in October and only OCP uniforms (doesn't apply to TA-50 or issued gear) are authorized for wear.

I can’t find anything in 670-1 that specifically prohibits this, but I would infer its prohibition from DA PAM 670-1 4-7. o.

The Army has not manufactured a great deal of combat pants in OCP at this time, so there is a good chance that those pants were OEF-CP, which is no longer authorized.

Good question. Can't think of a reason why someone would unless there is a unit policy against it.