Actually Verdi (the union) is also complaining about some of the working conditions as well, which they consider inhumane. It is mainly, but not exclusively about the wages.
> for a decent wage
Amazon is not paying according to the Branchentarifvertrag (industry sector collective agreement) and refuses to even sit down with the union and talk about collective bargaining. They are also actively trying to keep their employees from unionizing. If you consider the current wages "decent" is up to your point of view, but the union and I'd guess most of the affected workers disagree.
> The fact that most of the workers are not on strike proves that they're satisfied with their jobs
This proves nothing, in particular not job satisfaction, and is actually also not how things work in Germany and therefore just wrong. First a union will usually selectively shut down just a few facilities instead of all at once. This is called "Warnstreiks" (warning strikes), and this is what is happening right now. Only later and and only in accordance with German strike/bargaining law - e.g. you have to be unionized and are prohibited to violate the "Friedenspflicht", which regulates when and under what circumstances a unionized strike may happen - it is possible to enter the actual "Streik" where all (unionized) employees stop working. Plus, amazon employees where often not unionized before this started, so it might take some time and convincing for the union to get employees to join before they enter the "Warnstreiks".
> Plus, nobody forced them to accept those jobs
Well, the "Arbeitsagentur" (job center) can cut either your unemployment pay or later social welfare (Hartz 4) if you refuse to accept the jobs that are offered to you. If you also happen to have a family, and IIRC amazon employs quite a few single mothers for example, then you're "forced" to take the job in a way.
Some people, in particular union folks of course, even accuse amazon of predatory behavior, especially seeking out the weakest members of society for their work force because they are the least likely to fight against inhumane conditions and low wages. I used to think this predatory characterization was sensationalist and unfair, but by now, from what I read about amazon so far in the press and even in their own press statements, I think there might be something to it. But then again, I'm also just one of these Euro-socialists who thinks that everybody should have health insurance and should earn a living wage if working full time. Amazon more or less takes the same stance as you seem to take: They claim that they are just helping the weakest people. Treating your low-paid work force as if they were industrial robots (one of the complaints by workers and the union, and more or less admitted by amazon to happen as a result of their workplace rules and "processes") and keeping them from effectively complaining about it by keeping them from organizing suggests otherwise.