Besides, the limit of bandwidth is almost entirely artificial. Yeah, it costs some amount of money to send, say, 500 SMS messages, serve 1 GB of data, or process a 5 minute phone call. But not nearly as much as you're paying and it's not like they pay per SMS/GB/minute, once the infrastructure is there they pay a fairly flat amount to keep each service area running (until it's time to upgrade but depending on where you are, that might even be publicly subsidized). So, whether you're allowed 10 GB or 20 GB per month makes barely a dent on their cost, but getting you to pay $15 for 20 instead of $10 for 10 when $20 for 20 already isn't an option is really good for their bottom line.
But it's not Hanlon's Razor, it is absolutely malicious. They want to make the process complicated and frustrating so you give up.
Besides, the limit of bandwidth is almost entirely artificial. Yeah, it costs some amount of money to send, say, 500 SMS messages, serve 1 GB of data, or process a 5 minute phone call. But not nearly as much as you're paying and it's not like they pay per SMS/GB/minute, once the infrastructure is there they pay a fairly flat amount to keep each service area running (until it's time to upgrade but depending on where you are, that might even be publicly subsidized). So, whether you're allowed 10 GB or 20 GB per month makes barely a dent on their cost, but getting you to pay $15 for 20 instead of $10 for 10 when $20 for 20 already isn't an option is really good for their bottom line.