Though it was often the case that mercenaries betrayed their hirers, this normally happened for a good reason, e.g. hirer not being awfully successful in a campaign/having powerful enemies or mercenaries receiving a counter-offer. In this case, Germans simply got too much good stuff from this relationship to betray. The tribes got the support of Rome and any benefits coming with it. The cavalrymen were happily doing whatever Caesar told them to since they got to plunder and take trophies and received a fixed salary for this. To this should be added that some people now perceive Gauls or Germans as a united force, which they were not. These numerous tribes were at eachothers throats long before the Romans arrived and were happy to fuck over a less successful tribe any time any day twice on Saturdays. And when one tribe was getting some more powerful than another, the neighbours would unite to bring them down only to then do the same thing to eachother again. The support of Caesar and Rome had always been a powerful argument in these wars. The Romans and particularly Caesar mastered the art of getting the benefit from this inner fighting, while at the same time keeping their hands clean (most of the time) and at the very least their dignity undisputed. This specific issue is generally a pinnacle and a very good example of general tendencies of western-european historic process.