If nothing else it's a testament to why graphic designers who don't have a particular interest in road signs shouldn't be allowed to design stuff like this. Forget just the US shields, every shield is stylized at best. Seriously guys, there are standard specs for these things which are publicly available for free, you don't need to redesign your own version of them!
It would be awesome if they actually used the standard design for state and provincial routes rather than ovals and home plates, but I've yet to see anyone but a state DOT or a roadgeek produce such.
The standard designs might work in a digital "large print edition" tile map or on well designed printed maps.
In the tile maps like Google, the more complicated shield shapes, like the US highway shield, cannot be rendered so accurately when they must be represented by a graphic with only a few pixels in each dimension. The same is true of shield designs where the most important part -- the route number -- is a rather small fraction of the area of the shield (e.g., Idaho, NC, MI).
For digital maps, you primarily need the route number to be a legible size to serve its functional purpose and for it to be inside a shield that is small to keep clutter low. I would expect that using accurate state highway shields in states like Idaho, Florida, Michigan, or North Carolina (to name a few) would be a poor choice for standard tile maps.
The US shields and oval US state highway "shields" are reasonable compromises, even if they are not perfect.
I'd love to see Google use a different font for the text in their maps so that the pairs "i","l" and "rn","m" are much easier to tell apart.