Got me confused for a second, so I checked with 3 of my computers at home and LEDs did not come on even for a second when polarities were reversed. Spend most my life experimenting and working with electronics (no PHDs and stuf but was instrument technician with few years at aircraft instrumentation maintenance) and never managed to have an ordinary LED to light up with DC polarity reversed, however at some frequencies with square wave AC they will light up, also there are some tri-state LEDs that work in both directions and with AC too but with different color for each state. So it is not impossible that in some circumstances they will work as described, it's just that mine did not. By definition LED is diode and I used them as diodes in ordinary way, light or not. I also worked with and at computers since '70ies and was radio amateur since grade 4, 1959, so I gues I have some credentials there too. So to conclude, normal, ordinary LEDs do not light up with DC polarity reversed, that's why they come with one lead longer or otherwise marked so polarity is known without testing.
PS. Oh yeah there is a way to have them light up with polarities reversed but it involves using rectifier bridge so it is not practical for very small packages.
MBs tested on are:
GB 790xt-USB3
GB 870a UD3
Asus m2n-CM DVI.
I know for sure that LEDs with reversed polarity did not work on any older MBs I ever used (many a time I accidentally reversed them) but it may be the case with newer ones although this is first time I ever heard of such cases.