This is because the tracking pixel is still included in the email even with no other "pictures" deliberately embedded. I'm not convinced that the email becomes spammy because of this, though, provided the rest of the email stands alone: many legit person-to-person emails have an image in the signature. In fact the majority of B2B emails I get have an (ill-advised, but innocent) logo attached to them.
Someone will probably chime in about whether it has become possible to turn off the pixel. Last I remember it wasn't possible, unfortunately.