Mike,
You are absolutely correct. The medical community definitely is averse to adults wearing diapers. Diapers are considered to be the last and least desired option for urinary incontinence. Your urologist probably considers your wearing a diaper to be his personal failure - he has not been able to successfully "cure" you. When I tried to educate one nurse - a pelvic rehabilitation therapist - she responded "I spend my time trying to get people out of these." She had little interest, but was better than most. Some medical practitioners also may be concerned that, when many people wear a diaper for any substantial length of time, they become more dependent on using the diaper and may become less responsive to other treatments, such as medications.
Re bedwetting web sites, most are honest and are just selling products designed to keep you from having to wear diapers. Some are genuinely concerned that, if you wear a diaper to bed, it will be easier to just give in and use it. Some also are concerned that a diaper could get in the way when you wake up (to your alarm) and rush to the toilet, and contribute to having an accident.
Most doctors and nurses cannot inform you knowledgeably about using diapers to manage incontinence because they simply know very little about them and are not qualified to offer advice.

They are most familiar with cheap institutional diapers used for only a very temporary period. A couple of doctors have told me that they just did not receive any training along these lines in medical school, although they were open to learning more.
--John