I don’t see a difference in principle between the two cases, although there are practical differences: more people (at least in the yeshiva crowd) hold the eruv to be passul than hold CS to actually be assur, and not holding of the eruv in some cases has ramifications d’oraisa.
Ubiqitin, a talmid of R’ Belsky, who holds the Brooklyn eruv is absolutely passul, could not be mesayaia someone to use it, but could be mesayaia someone to eat CS. Although he holds it’s a chumra worth keeping, he holds meikar hadin it’s muttar. There would probably be a chumra in lifnei iver not to give him CS, though.
OTOH, it would be lifnei iver to give CS to someone who is makpid, even if you hold it’s muttar. If he holds it’s assur, it’s shavei anafshei even if it’s muttar. Even if it’s just a chumra for him, it would still fall under the category of a bad eitzah, also lifnei iver.