There is no single set of rules, especially for cash games, but it is against the basic RRoP rules:
Quote:
POKER ETIQUETTE
The following actions are improper, and grounds for warning, suspending, or barring a violator:
[...]
Softplaying by refusing to bet against a certain opponent whenever heads-up.
Having said that, it occurs in most every room. Usually in a low stakes game where it's the same people playing against each other every day. Almost always a bunch of old guys who are there for entertainment and passing the time rather than "playing real poker".
In a room where there's only 1 table and the same guys make the game each week, it's unlikely the room is going to do anything to upset the apple cart. So learn to live with it, or play somewhere else. You can make your displeasure known, and even point them to this rule in RRoP (if they have a rulebook, it's probably in theirs too), but it's not likely to change anything in a room that small.
On the positive side, these guys are almost never trying to overtly cheat you. But as psand mentions above, it causes a number of other problems which can impact you negatively, if non-purposefully.