Thanks for the information, everyone! I see our friendly mods have moved this into the ATC section from the Spectators' Gallery - I wasn't sure which section to post it in!
I understand this a lot clearer now, and as to why a commercial aircraft may refuse a 'land after' instruction.
As to crews misinterpreting/mis-hearing the instruction, I remember an occasion when an Eastern Airways Jetstream was instructed to 'land after' company traffic, and the pilot read back 'cleared to land'. The tower controller immediately corrected him, but my instructor commented to me 'It's the same thing, for goodness sake.' Even with only half a dozen PUT flying hours, I knew the two instructions weren't the same and had an idea that 'land after' meant that the responsibility for separation was with the PIC and not ATC. It's somewhat concerning that an experienced pilot would consider incorrect readback of this nature to be a nothing issue!