Every vendor that offers fixed price (for unlimited data) is going to have various techniques to throttle your use, to stop the 1% of people who have hundreds of terabytes from getting an unfair deal. My first cloud vendor (Mozy) began aggressively throttling your bandwidth around 500gb and as you passed it and approached 1TB, it became essentially impossible to back up more data. It was never down, just so slow you could not keep up with even a modest change rate.
At least Backblaze is up front, most lie about it (Mozy did). Though Backblaze does not highlight their rule enough.
I've always felt like I was better off picking a better business model where I paid per gigabyte. My current vendor is also Backblaze, but with their B2 service, which is essentially storage-for-a-fee. You bring your backup program of choice (I use Cloudberry, now called MSP360 I think). This has the added advantage that you can encrypt the data with keys that Backblaze does not know, so if they are ever hacked your data is relatively safe from access.
All-you-can-eat buffets for food is inherently limited by the human stomach -- which at times is impressive, but still has limits.
All you can eat data can be massively abused, and so vendors must have mechanisms to stop it. Some are limits, some are just policies (if you use to much we can terminate -- usually with "too much" not defined), some are more creative (like Backblaze limiting offline storage which is where most people keep massive amounts of data).