They can use Cassano because Sampdoria are no longer in the EL, they can't use Van Bommel.
You are most probably right, but reading the CL 2010/2011 regulations, I'm not sure if I understand them well and if Cassano can play or not (about Van Bommel it's clear that he's cup tied).
Here:
18.07 As a rule, a player may not play in a UEFA club competition (i.e. UEFA
Champions League and UEFA Europa League, but not UEFA Super Cup) for
more than one club in the course of the same season. Exceptionally,
however, a player who has been fielded in the first, second or third qualifying
round of the UEFA Champions League or UEFA Europa League is entitled to
play in the UEFA Champions League or UEFA Europa League for another
club as from the group stage, provided that his former club did not qualify for
the group phase of either the UEFA Champions League or the UEFA Europa
League; moreover, as from the start of the round of 16 (round of 32 for UEFA
Europa League), a player can be registered in accordance with paragraphs
18.17 to 18.20 below. For the avoidance of doubt, a substitute player who
has not been fielded is entitled to play for another club competing in the
UEFA Champions League or UEFA Europa League in the course of the
same season, provided that he is registered with the UEFA administration in
accordance with the present regulations.
18.17 For all matches from the start of the round of 16, a club may register a
maximum of three new eligible players for the remaining matches in the
current competition. Such registration must be completed by
1 February 2011 (24.00 CET) at the latest. This deadline cannot be
extended.
18.18 One player from the above quota of three who has played UEFA club
competition matches for another competing club in the current season may
exceptionally be registered, provided that the player has not been fielded:
− in the same competition for another club; or
− for another club that is currently in the same competition
-----------
Now, if Sampdoria played in the third qualifying round, then there are no problems, but didn't Sampdoria play in the 4th qualifying round?
In that case Article 18.18 is valid and since at one point Sampdoria played in the same competition with Milan (Champions League) then Cassano can't play for another team that is playing in CL.