OFP's modeling of the LAW AT (Anti-Tank) weapon is indeed not realistic. Real LAWs are not meant to be reloaded (though I've been told it is possible - don't try this at home). Normally, real LAWs are fired once and tossed away (please recycle). OFP's LAW can be reloaded like all other AT weapons in OFP. This is simply a design inaccuracy on the part of BIS. Forgive them.
Or maybe not. OFP player ReverendTed points out that, at 5.5 kilograms each, soldiers can easily carry 3 LAWs. It could very well be that it was too much work for BIS to model a visually varying number of LAWs remaining on a soldier's back after each one is taken in hand and fired.