I think its stems down to the fact that territory north and east of the Rhine River largely becomes a no-mans land due to the nuclear attacks on NATO targets in Germany and the Netherlands, which Belgium avoided when it joined France in officially withdrawing from NATO and declare their neutrality after American, Canadian, and British troops crossed the East German frontier in support of the West Germans. Although the Flemish north of Belgium has some sympathy for their Dutch brethren in the Netherlands, in hind sight they are probably glad that the Belgian government threw their lot in with France and were spared the nuclear destruction of their country.
Belgian units serve alonside French units in occupying territory west of the Rhine to secure a geographic barrier and guarantee its frontiers against the hordes of refugees and renegade military personnel which were swarming across the border. From T2K Module: Going Home their are plenty of Belgian units serving alongside French and even Senegalese troops. Belgian troops of Flemish origin who are sympathetic to the Dutch may be transferred to the German border or other units in the Franco-Belgian Union.
The French III Corps
2nd French Armored Division:
501st Tank Regiment (armored)
6th Cuirassier Regiment (armored)
1st Belgian Guides (armored)
5th Infantry Regiment (infantry)
30th Chasseur Group (mechanized)
Chad March Regiment (mechanized)
1st Marine Artillery Regiment (155mm)
5th Engineers Regiment (engineers)
3rd Belgian Chasseur Regiment (cavalry)
17th French Armored Division:
2nd Belgian Guides (armored)
1st Belgian Lanciers (armored)
2nd Chasseur Regiment (armored)
5th Belgian Infantry Linie (mechanized)
1st Chasseurs Ardennois (mechanized)
151st French Infantry Regiment (mechanized)
2nd Belgian Chasseurs (cavalry)
17th Artillery Regiment (Belgian 155mm)
1st French Armored Division:
1st Cuirassier Regiment (armored)
6th Dragoon Regiment
3rd Cuirassier Regiment (armored)
8th Chasseur Group (mechanized)
16th Chasseur Group (mechanized)
1th Belgian Infantry Linie (mechanized)
1st Spahis Regiment (cavalry)
9th Artillery Regiment (155mm)
13th Engineers Regiment (engineers)
III Corps Troops:
76th Infantry Regiment
15th Senegalese Tirailliers Regiment
58th Regiment Artillerie a Pied (Inf)
2nd Hussar Regiment (Cav)
4th Chasseurs Ardennois (Cav)
8th Marine Parachute Regiment (Para)
Belgian Parachute Commando Regiment (Para)
31st Engineer Regiment (Eng)
71st Engineer Regiment (Eng)
16th Artillery Regiment (155mm)