As far as a restaurant is concerned, why would they really care? It might annoy customers, but as long as the tables are still filling up, I feel the restaurant is more than happy, because they're still making the best of their available resources (tables and wait-staff).
You answered your own question -- to the customer, it's annoying.
... and to the restaurant owner, it discourages walk-in traffic.
It's immoral in that it's dishonest (We have a word for reserving space at the restaurant by claiming to be a party of two, but not being one -- it's called lying). If you wanted to do this in the moral clear, you need to speak to the restaurant owner first, and see if they'll pre-allocate you tables.