I wonder if they are bullshit. Hear me out, I agree on the surface they don't make sense.
The company is selling a service that is shared among its users. Let's say for argument, they have a 100MB pipe. They could limit this to 4 users at 25MB/s each, but to break even, this would cost each user $300 per month. OK, competitively that doesn't make sense. So they watch user patterns, and see that a typical user wants 50MB/s, but only uses the 50MB/s twice a day, for about two hours.
So, based on this typical pattern, they come up with a new plan - you can have 50MB/s, and because I am guessing you will only use it 2 hours a day, I am going to oversubscribe the channel by 10X. If everyone acts in a predictable pattern, I can deliver the 50MB/s rate and charge 1/10th the price - $30 / month.
Now, I have a great service at a great price. Along comes a guy who has a need to pull all 50/MB/s for 24 hours a day. Oops, that blows my whole model - I am now way over subscribed, and providing poor service. What could I do about this? Well, I could put a total cap on the usage, so you can really only use about what I expected - 50MB/s for a couple hours a day. You can't be a data hog, or if you are, you have to pay for it.
To me this is analogous to people who complain about ads on cable TV. The argument goes - I am paying for Cable, I shouldn't see ads. No, that is close, but not right. A cable company could charge you enough to pay for everything, they could pay for everything through ads, or they could split the difference, and charge you less and show fewer ads.
So, TLDR: Data caps are ridiculous if we are paying for dedicated bandwidth. I don't think most of us are, we just assumed we are. I think we would be shocked at what the price would be for dedicated bandwidth.
Full disclosure - I don't work for an ISP or cable company. I have no idea if their profit margins are reasonable or not, I don't think that is actually relevant to the argument of whether data caps are reasonable or not. That goes to the argument of whether the price including caps is reasonable.