They do not. However, with more and more dictation being stored on computerized digital dictation systems, the time has come to adopt a unit of measurement which supports, rather than punishes medical transcriptionists. Because it is based on a wildly inaccurate standard and is a phenomenal waste of time. A digital dictation system will measure the length of each report to the 10th or 100th of a minute. Everyone tells time by the exact same standard.
Billable time is also a kind of billing that is universally understood.
When people get their long distance bill, they understand that the phone company does not care how many words or pauses are contained in each conversation. All they care about is the length of time that the two parties were connected. When people pay for on-line services like Genie, Prodigy or America OnLine they pay for connect time. The service they use doesn't care how many key strokes they make or how many files they download. All it cares about is what the computer's clock registers as the client's on-line time with the service provider.