Shield is the MRDS company in Europe. Just take a look at how many Shields were in IPSC European Handgun Championship Production Optics.
Glock doesn't care about the US civilian market, they already sell every gun they make. If they want to expand in the European market it makes sense they make the gun to fit the most popular optic in Europe.
Glock also makes guns factory milled from RMRs. Those would sell like hit cakes here in the US. But why would they right now when the down right terrible MOS is flying off the shelves.