Sidenote: "As in screenshot" is a bad description. It cannot be searched, and even while pictures are worth a thousand words, they neither tell what you expect instead, nor why.
This happens to any unit without a weapon. Locomotor doesn't matter.
Infantry without a weapon gets "unable to enter" cursor over an allied Grinder=yes structure. Engineer logic seems to override this behavior, making it able to enter.
Other infantry and units get "unable to move" cursor over a Grinder=yes allied structure.
If you can send any unit to an ally grinder, why should it be different for engineers and subterranean units?