A fair question.
It has been awhile since we implemented the various signature restrictions, but I'm going to guess that the font restriction was put in place to prevent the use of fonts that might appear larger than the standard font (thereby skirting the size restriction to some extent).
Having looked a little more closely, I'm convinced that this isn't an issue and have rescinded this restriction (in the interest of full disclosure, it was probably me who made the oversight in the first place
).
Thanks for pointing this out
Matthew.