They can include or exclude any transactions as they see fit.
Then this is the culprit that needs to be fixed in a future update. Miners should have no say whatsoever in which transactions to in or exclude.
Unfortunately, Satoshi's breakthrough, that made bitcoin possible, was to give all the power to the miners -- with the proof-of-work trick to keep them from cheating. That would have worked if mining had remained well-distributed, so that it would be practically impossible to convince a majority of them to sabotage the system.
But it did not happen that way, basically because the price shoot up to 100 or 1000 times what it should have been, given its usage. With that hyperinflated price, and the fixed block reward, mining become a very profitable activity, that was worth carrying out in an industrial scale, by entities distinct from the users. Then the mining industry got concentrated in a few companies because of economies of scale.
Bitcoin was created to be a peer-to-peer payent system that did not require trusted third parties, including central authorities. Strictly speaking, bitcoin is broken right now; because the top 5-6 miners are third parties that must be trusted not to abuse their power. With the BIP100 discussion, bitcoiners seem to be gradually becoming aware of that fact: it will be the miners that will decide whether, when, and how to change the block size limit
Bitcoin may still get "cured" if mining becomes again distributed among the users. However, I do not see how that could happen, unless the price crashes to such a low level that no one will want to mine for profit, and mining becomes again a client activity -- say, a convenient alternative to buying bitcoins, that an ordinary person could use to get some bitcoins to pay for coffee or whatever.
Taking away the power from the miners -- in particular, forcing them to process all transactions issued by clients -- would require reforming bitcoin to the core. It seems that another Satoshi-level ingenious idea would be needed do that without introducing some trusted central authority.
[/quote]