scan XP becomes not worth it if you spam the scans in the same DAY, as explained in the manual
http://game.planetarion.com/manual.pl?page=formulae ->XP for scanning.
What comes to abuses, there's a quite clearly written portion under terms and conditions, available at:
http://game.planetarion.com/terms.pl , out of which the most fitting would be: "Abusing game features in order to gain excessive XP (e.g. roid farming)".
I'm not taking a stand if it's against the rules or not and who defines excessive tho', it's a strat... is it "legal" maybe, maybe not, guess it's up to the community to decide.