No that's a pretty good question actually!
The short answer is that if you want an overall constant shift, you can do this:
ampo += -0.5/4,"Id",1
and importantly do that outside the loop so it's only a single shift. If you put that inside the loop if would be equivalent to adding a shift
ampo += -(N-1)*0.5/4,"Id',1
unless that's actually the behavior you want.
On a more big picture level, recall that a constant shift to a Hamiltonian really means a term which is the tensor product of the identity operator on every site. The way OpSum works when converting to an MPO is that any sites not included in a term are assumed to have the identity operator acting on them. So a term like "Sz",3 is really Id1 Id2 Sz3 Id4 Id_5 ....
We should add as a feature, though, the ability to just add a constant by itself as you tried. It's a nice suggestion and could be convenient in many cases.