Not my experience. With o2, the sales guy on the phone even recommended buying a fritz box when I told him I don't want to rent their "o2 homebox" (which is not made by avm and has a terrible reputation) for 2€ a month.

With Kabel Deutschland the process was also painless. I disconnected their box which they lend to you for free (also not avm), connected the fritzbox I bought on ebay and ended up on some form of captive portal where I had to enter my customer information and boom I was online. I've yet to encounter an isp which tries to force you to use a fritzbox. But then again I wouldn't mind since they are reliable and stable. :)