I've explained this a bunch of times this season already, but I can do it again heh
Leagues can now have two settings. One says, "I only want to see guys that have been put on a major league roster". That is the option for people who want consistency. Some people using that option now are confused because they are seeing guys like Strasburg. That is because he is on a major league roster today. Consistency means just that; it involves no judgement about whether they are likely to stay on the roster, whether their team should have put them on the roster, etc. etc.
The second option says, "I want to see any player that Yahoo! deemed interesting." Choosing that option gets you earlier access to prospects and so on, but necessarily means that it will be subjective and you may or may not always see who you want to see right when you want to see them. It can include prospects we think people want to add, veterans that haven't yet signed on with a team, etc. The players that are being forcibly included in this setting can be seen using thehttp://baseball.fantasysports.yahoo.com/b1/
url that I mentioned earlier. The fact that Heyward isn't in this player pool could be because we're "stupid". Or it could be because it is ages until the season starts and players are still being added. Note that I've already posted that there is another batch of prospects coming to the player pool soon, but feel free to go with the "stupid" explanation if you like. I, personally, am about as bright as a 3 watt light bulb.