My guess is this is a gap in the reverse engineering Yongnuo does for the electronic communication protocol of the Canon hotshoe. The AE-1 is old enough that it's using the film version of TTL/A-TTL, while the Rebel 2000 uses E-TTL, and the T1i uses E-TTL II. Canon flashes can probably detect and switch to the different modes for TTL/A-TTL, E-TTL, and E-TTL II, but maybe the Yongnuo flashes can only recognize the film TTL and eTTL-II signals or were only engineered to handle those two as the most-likely TTL scenarios (or the only ones where YN had gear to test against).

As an example, I have a YN-568EX flash. It works great on my 5DMkII and 50D. However, when I put it on the hotshoe of my Powershot G9, it fails to fire. The G9 is clearly using some variant of the regular EOS hotshoe communication protocol, but it doesn't do E-TTL II. My 580EXII works fine on all three hotshoes, and can even have its manual power level set from the G9. It's clearly been designed to recognize my Powershot variant. The Yongnuo has not.

see also: The EOS Flash Bible's sections on the Canon TTL flavors.