Apparently, Citrea decided not to put their spam in inscriptions because inscriptions have to be "unwrapped" in a second transaction. When the data is less than ~150 bytes, it's cheaper to put it in unspendable outputs, despite getting no witness discount.

I think they could get the witness discount and still avoid a second transaction by putting the spam in the control block. It lets you add up to ~520 bytes of spam by encoding your data as merkle leaves, each of which can be 32 bytes. No extra tx required, AND you get the witness discount.