Why would they do that?
What would they gain?
by only allowing a select few tx's in does2 things:
1. reduces the data they have to handle when mining
2. choose the tx's that have fee's added and ignore the rest
I have heard of "selfish mining" where a pool has solved a block but did not publish in order to get a head start for the next block(s).
Anything similar to the above?
not really. but yea.
not only are pools pre-selecting profitable tx's, but they can (try atleast) to mess with other things like trying to process the next block before other pools to always remain ahead. amungst other selfish tactics thats not really beneficial to the general community.