By throttling the speed they are limiting how much data you can use. Not unlimited.
With due respect, everyone who honestly believes that "unlimited" means "unlimited" simply isn't realistic, reasonable, or paying attention.
ALL unlimited plans for ALL services are done based on a reasonable expectation of usage. Fair or not, that's the way it is. Every service provider of every type that says they offer an unlimited plan is over-subscribing themselves. Obviously, there aren't unlimited resources anywhere, and they can't provide to customers that fully leverage "unlimited" plans. So they guess. They estimate. They collect data and try to provide the most reasonable expectation of how much the average consumer will use, and cater to that, and deal with overages as outliers. When overages become the norm, they need to adjust.
You can argue that the verbiage shouldn't be "unlimited" but consumers do this to themselves. The word "unlimited" is put onto a marketing literature and everyone wants that. So all the manufacturers, to stay competitive, say they offer "unlimited" plans even though they aren't.
AT&T put in a tier and all of a sudden everyone freaked even though it was going to save 90% of their customers $5/month.
Your unlimited plans are nothing of the sort, and, being rational about it, cannot be unlimited. Tiered pricing is what needs to happen - be it speed or consumption tiers. You can't buy "unlimited" electricity. Why should you be able to buy unlimited data? It should all be metered.