It is supported in WPC:
It's just not displaying quite like in a desktop Wolfram Language product.
Compare with Mathematica 9:
In[1]:= Inactive[Length][{a, b, c}]
Out[1]= Inactive[Length][{a, b, c}]
In[2]:= Activate@%
Out[2]= Activate[Inactive[Length][{a, b, c}]]
In[3]:= $Version
Out[3]= "9.0 for Mac OS X x86 (64-bit) (January 24, 2013)"