There is absolutely nothing preventing an ISP from offering a conventional proxy server for those users who want it, instead of "hijacking" everyone's traffic behind their backs.
True. However, most users wouldn't even know how to set a proxy in their browser anyhow, so the advantages, both in speed and cost, are largely lost when an ISP does this.
Emotive terms such as "hijacking" and "behind their backs" don't seem relevant to the discussion. They are doing something which assists their whole user base because the majority are too dumb to do it themselves. Having used ISPs with transparent proxying for years, I believe there is only one important potential downside: The transparent proxy devices must have redundancy and 100% uptime.