Why do tunes grey out on my iPod?

This should be a simple enough question, and it's exactly as it says. WHY do tunes grey out on my iPod?

I have three of them, 1st, 2nd and 5th generation. The earliest one doesn't have wifi so works with a Bose Sounddock, the second one does and is my main device for music - why, since I have a later model?

Simply because I use them in my workplace, which is not at my home. I buy CDs (yes, real ones!) and copy them to iTunes, and then usually to the iPod. Many of these CDs I have had for years.

I go out to the garage, turn the latest iPod on and a lot of the songs are greyed out. WHY?

I apparently haven't synced the iPod in a while - why should this matter? Surely if I have music on it then it should play what is on it until I want to change it. I shouldn't have to back online and beg with some program or other to please let me play music which I've paid for. If it needs a software update - which all seem to be about 1.3Gb - then it should wait until I have time to do it, not hold me to ransom until it gets one. It's now lying in the drawer, out of power, and can stay there. I just see it as a waste of money and a botheration.

So: why do tunes disappear off my iPod, then I have to go to some mystic Cloud that I don't need or want to get them back, even when they're my physical CDs in the first place? How do I stop this without it constantly reminding me that wi-fi is turned off or I haven't updated it in the last 5 minutes?

Incidentally the 2nd generation one doesn't do any of this and so it gets used - the tunes are on it and they stay on it. Did I waste my money upgrading to a new device that's useless for me?