> Have seen a couple of sans...

Hmm... can you tell us, which ones?
Can't remember of any that fits your description...

I can give Scala sans and Interstate as examples...
I think they theoreticly should have more contrast on these characters...

I'd love to see the fonts you're talking about.

> Is this legal?

I like doing it too, so I certainly hope not! ;-)
Most recently, my Mana-16* has stroke contrast, but in those letters the thinning happens at the joins.

* http://www.manic.com.sg/blog/archives/000121.php

Stroke contrast can be a good thing (especially for readability), irrespective of where it comes from. As a result, there's really nothing Wrong about doing that, except upsetting some type designers.

The "theory" might be that it makes structural sense, that it has a harmony missing -or at least different- from the doddering ductus school.

hhp

One argument against this is that all other characters contrast are based on calligraphy, and this makes the font uneven. But then again, who cares as long as it looks good...