That is an interesting question, and out of interest I had a quick look.
It seems it was introduced quite a long time ago: trytond: dd2fb553a4b6 (and perhaps this behaviour was there before then, but this is where the ir/sequence.xml file was introduced…)
It is probably a legacy from TinyERP. I guess it was because at that time it was quite complicated to show inactive records. I think this could be safely removed.