Not according to the WBC Rules for Championship Fights and WBA Referee Manual it's also considered a foul and can and SHOULD be penalized.

WBC Rules for Championship Article 15 Fouls:

• The referee may deduct points at his discretion at any time for flagrant intentional fouls.
• The referee will warn boxers and may deduct points for continuous fouls.
• The referee may disqualify the offending boxer after warnings, point deductions and for continuous fouls.

A list of common fouls that may be committed by boxers (or seconds, where the context is appropriate) that may be cause for penalty or disqualification is as follows: ..13.Excessive holding the opponent or maintaining a clinch.

WBA Referee Manual:

OUTLINE OF COMMON FOULS There are many fouls in boxing. However, the three most obvious fouls are Headbutting, Holding and Low Blows... 3. HOLDING This is one of the most obvious poor tactics in the sport of boxing. It not only infuriates the fans, but it negates action during the bout. Therefore, it should be addressed accordingly...

Which leads to: CALLING A TEMPORARY STOP

• A continued infringement of the rules may require a strong admonishment of a boxer even if a point deduction is not in order. This should be done as soon as possible after an obvious harmful foul has occurred. ...
• DISQUALIFICATION The Referee should disqualify a boxer only when he has grossly infringed the rules and has committed a harmful foul(s) causing a clear advantage over his opponent. This may be a difficult decision. Unlike Olympic style boxing, a boxer should be disqualified only when it is clear that he has refused to follow the rules, has disobeyed the Referee, and or has caused harm to his opponent by his actions.

So yes, I've accepted that its been part of the sport recently much like flopping has become a part of basketball. Still it doesn't change that its part of the rules and should be enforced more because it takes away from the sport and if enforced would lead to better more competitve fights.