Here's a MWE that produces the oddly distributed columns:
\setuplayout[backspace=0.5in,width=4.5in,topspace=0.5in,top=0in,topdistance=0in,
header=0in,headerdistance=0in,bottom=0.25in,bottomdistance=0.125in,
footer=0.25in,footerdistance=0.25in,height=7.75in]
\setuppagenumbering[location={footer,middle}]
\setupwhitespace[quarterline]
\setupbodyfont[pagella,9pt]
\definepapersize[digest][width=5.5in,height=8.5in]
\setuppapersize[digest]
\starttext
\startplacetable[location=force,title={Stun Levels}]
\definecolor[back-3][r=.8,g=.8,b=.8]
\setupTABLE[background=color,frame=off,framecolor=white]
\setupTABLE[row][odd][backgroundcolor=back-3]
\setupTABLE[row][1][topframe=on,bottomframe=on,framecolor=black,rulethickness=0.75pt]
\setupTABLE[row][4][bottomframe=on,framecolor=black,rulethickness=0.75pt]
\bTABLE[width=broad]
\bTR\bTD {\bf Damage >\\Soak by:}\ \eTD\bTD {\bf Stun\\Level}\eTD\bTD {\bf Stun\\Effects} \eTD\eTR
\bTR\bTD 0 or less\eTD\bTD Unharmed \eTD\bTD None. \eTD\eTR
\bTR\bTD 1 to 8 \eTD\bTD Stunned \eTD\bTD -1D on all actions for 2D rounds. \eTD\eTR
\bTR\bTD 9+ \eTD\bTD Severely\\Stunned\eTD\bTD The character is knocked unconscious for 2D minutes.\eTD\eTR
\eTABLE
\stopplacetable
\stoptext
Naively, I expected the third column, which has the longest text, to absorb more of the width, rather than the three columns getting about the same width.
I added the following two lines before the \bTABLE:
\setupTABLE[column][1][width=6em]
\setupTABLE[column][2][width=6em]
This caused all the space not used by the first two columns to be used by the third column.
Thinking about it a little more, the first result makes sense: there is probably no way for ConTeXt to figure out that the third column should be wider unless it is given some constraints.
Interestingly, groff's tbl has a Column Specifier, x or X, that, after computing all the widths of columns without an x specifier, uses the remaining line width for the columns with an x, distributing them equally across the columns with x specifiers. (I often use groff for quick-and-dirty documents, or where ConTeXt is not installed; I use ConTeXt for documents that are more complex, or ones that need their appearance customized.) This is particularly useful when one column has paragraph length text contents, and the other columns don't.
I don't suppose there is a bTABLE equivalent? Or an equivalent in one of ConTeXt's other table types?