We ran into the same problem. The css designer was using this style:
style.css
img {max-width: 100%; }
Instead of disabling the zoom control, we fixed the problem by overriding the img style for map_canvas elements like so:
style.css:
#map_canvas img { max-width: none; }
The zoom control now displays correctly.
Setting “img max-width:100%” is a technique employed in responsive/fluid web site design so that images re-size proportionally when the browser is re-sized. Apparently some css grid systems have started setting this style by default. More info about fluid images here: http://www.alistapart.com/articles/fluid-images/ Not sure why this conflicts with the google map…