That is a function of how the themes designer coded his/her theme and not anything that comes from WP.com.
For reference, most theme designers make the category pages (and monthly archive pages) display this way for brevity sake.
If a reader calls up a category archive that contains a large amount of entries having pics displayed and the full post would be very slow loading. In most cases the pics are stripped out and only an excerpt is displayed.
You can submit a feedback to the powers that be. But your request, if granted, would effect all users selecting the same theme. Whether or not it would be granted is a function of how many desired the same thing.