You are unable to purchase a SailRail ticket without obtaining a ferry reservation, so doing this is taking one of the limited number of ferry reservations away from someone who actually wishes to use the ferry. In addition to this, abuse of SailRail tickets risks their withdrawal in future, and their conditions have previously been tightened due to abuse.
SailRail is the only affordable way for people who cannot or do not wish to fly to travel between Britain and Ireland. It is also often the only affordable way for those who who need to travel at very short notice, at times of very high demand or with bulky luggage. My extended family were recently able to attend a funeral thanks to SailRail - having been quoted something in the region of £1,000 which they could not afford for next-day flights for two adults over the Jubilee weekend, they were instead able to travel by SailRail for under £200 return.
SailRail is not some obscure loophole that means you can travel via London on a £10 Glasgow-Edinburgh ticket, that everyone can have their fun with until it eventually gets closed and nobody's any worse off because it shouldn't have happened in the first place. It is a very useful product which is a lifeline to many, and its withdrawal would cause actual hurt to its intended market.