Many open android tablet devices

I believe I may have overstated the openness of these devices. What seemed to me at first as evidence of the code being available for them (alternate ROMs), was really just evidence of the devices not being locked down. However, it turns out that for some (most?), the source is not actually available. It is a strange situation, the manufacturers seem to still be releasing new ROMs with bugfixes, but they are not releasing the code. What a shame, I cannot see what they gain from this. Many on these forums would enhance the code and make the devices more valuable if they did.