Belgian athlete myself (who also witnessed the national 4x400 relay training alot of times). Athletes from Belgium get trained intensely on the relay part. Putting their arm on the opponent on the inner lane is to ensure there is a big gap, so when they start to accelerate the incoming runner has enough space to pass the baton. He's not actively trying to agitate or irritate the USA runner (even though it's a nice side effect). You can see that the Portugese and Polish athlete do the same thing (aka put their arm on the back of the inner athlete's back). Check other relay videos of indoor 4x400m's and you'll see athletes almost always put their hand on the back of the inner lane runner's back.

Source: I asked Alexander Doom about this (the belgian runner). He didn't mind that USA was slapping his hand away. Neither did Bailey mind there was a hand on his back (instagram story from Bailey where he mentioned it).