The courts get these eminent domain cases all the time. It's due process, but the property owner almost always loses.
I favor a reasonable amount of eminent domain powers for utilities, in return for regulations on the utilities that serve the public interest. Requiring the telephone companies to allow competitors to access their facilities is a reasonable tradeoff for eminent domain powers.
As far as the cable companies are concerned - their technology and topology is much different that the telephone companies. They don't have the physical capacity to carry as much signal as the phone companies because everyone is sharing the same wireline. In contrast, the phone company generally has a dedicated line from each home to the central office (at least for the homes within a certain radius of the C.O.), and switches for point-to-point connections.
The cable company has other regulations imposed on it, such as the "must carry" rule, that aren't applicable to the telephone companies.
The regulations have to fit the services offered by the utilities, so trying to impose telephone company regulations on a cable company doesn't make any damn sense.
But at 75-80% of cost? I don't mind the shared access issue. It's that they can, and do, run commercials that gloat that they don't have to pay their phone bill to us or that they can give the same service to the consumer cheaper is just too much.
And about sharing cable lines. That's still not that great of an excuse. Most phone resellers resell in name only. They could easily do the same for cable. All they'd do is send their check to a different company and they then send Cox, charter, etc a check for 80% the cost. And what of cable modems? Am I able to resell cable modem access at 80% of cost to residential customers as well as providing phone service over coax to business consumers? As it is most cable companies are dumping coax trunks for fiber allowing them more bandwidth.