That is exceptionally easy to explain.
Because the code is already there and what good would it do to remove it just for a single faction?
Correct, no one would expect them to change the code to remove the slot just for one army. However I wouldn't expect them to issue a rare skin for USF heavy tanks if they had no plans to ever use it.
One of the reasons would be: there was a total of 0 pershings during battle of bulge, which USF are based on.
First pershing have seen the combat only after the battle of bulge(and let me say this again, USF are based on US forces during Battle of bulge, not earlier, not later, exactly that, same for OKW).
Yeah, the base design is from that time period but that isn't something they absolutely couldn't deviate from. The new commander with the Pershing would be called "late war tactics" or something along those lines. Wow! that was an easy problem to solve, wasn't it?
Whooping 20 of them have seen any action.
Makes as much sense as including IS-3 tank, because one regiment have seen action on far east.
Well, that's 20 more than zero. It was a popular unit in COH1 and the war footage of it destroying a Panther in Cologne has made it a WWII icon. There would be no problem including it if balanced correctly.