Sure it is the decent thing to do to give them a reason, but also invites arguments so I'm of two minds.  They will almost inevitably disagree with your reason and get very angry.  Relay operators are humans and can't deal with angry people day after day, and being kicked off of a relay in nostr shouldn't be a big deal, you can just trivially go somewhere else.  If I was booted from a relay I'd get very excited and happy because "fuck you bitch I'm using nostr, and you can't stop me!"  and it would feel great to have such a retort.... It is like sometimes I wish a nuclear war would actually happen so I could feel good that I moved to New Zealand.... if nobody ever tries to censor me, what was the point of nostr?    So being explained to isn't really something I think relay ops need to do.  Maybe they just want to save on ISP costs (traffic, disk space).

Giving them a chance to download their data is also the decent thing to do, but probably not necessary in practical terms because ... well, because you are using lots of relays aren't you?

I recommend against posting to 17 or 25 relays only because it does not seem like the decent thing to do.  I don't plaster all the walls of your home with posters of my missing cat for the same reason, I just put it on a few telephone poles.