Do you mean this park outline: https://www.openstreetmap.org/relation/6431116 ? As far as I can tell it represents the name of the "park" that this part of the OLT is contained in, and actually is not the bike trail itself. In fact, "East Side Bike Trail" is listed as a park on the milwaukee county parks website: https://county.milwaukee.gov/EN/Parks/Explore/Find-a-Park

And yes, I do find this confusing, but  mapping can be confusing sometimes :)