it's really irrelevant what we think the rules should be, they've been the same since the year 2000 at least ... the argument about all U20's side should qualify as well as the A sides, and its tough luck if you don't have an A side, is flawed IMO, because it means that some unions end up protecting more players than others.
You can't tell me that someone at Ulster didn't know to check with the WRU or the IRB, or that the player's agents don't know this stuff ...
Here's the relevant section on the next 15 from the IRB regulations, regulation 8 ... as you can see, the next 15 remains the same for a period of 4 years, surely professional rugby players, player's agents, clubs, administrators, etc ... know which teams these are, or know where to find out
http://www.irb.com/mm/document/lawsregs/regulations/04/23/28/42328_pdf.pdf
6. How do I know which team is a Union's next senior fifteen-a-side
National Representative Team?
There should be no uncertainty over which team constitutes a Union's
next fifteen-a-side senior National Representative Team since, as from
January 1 2000, Unions are required to notify the IRB of the name of its
nominated next senior fifteen-a-side National Representative Team. The
team nominated remains the Union's next fifteen-a-side senior National
Representative Team for a period of 4 years. The identity of a Union's
next senior fifteen-a-side National Representative Team can be verified
with the Union concerned and/or the IRB.