Why are rules regarding the use of oxygen masks by commercial pilots often ignored?
The rule you've found is under part 135 of the FARs, which generally governs commuter and on-demand operations like many charter flights. Most commercial airlines, such as those flying large passenger jets on regularly scheduled routes, are part 121 operators, which follow different rules. 

For part 135 operators, there are a few caveats. First, above 25,000 MSL, there is an exception to the oxygen requirement if "each pilot has an approved quick-donning type oxygen mask." If the aircraft is so equipped, masks are only required above 35,000 MSL or when one pilot has left the cockpit. 

The rules for part 121 operators are a bit different and can be found in 14 CFR 121.333. Under these rules, if quick-donning masks are available and the aircraft has more than 30 seats or a payload capacity of more than 7,500 lbs, oxygen is not required at or below FL410 unless one pilot has temporarily left the controls. 

Since the majority of videos you may see online probably involve two pilots flying a large part 121 airliner (or one under a foreign regulatory regime, where different rules apply), oxygen masks would not be required in most of the situations where cockpit films are released. 

That said, I found this article, which may be of some interest: Study: Pilots Ignore Oxygen Regulations. It cites the results of a survey conducted as part of a student's master's thesis which concludes that the rules are ignored quite often and that there really aren't effective ways to enforce them either:


  The compliance numbers for Part 121 respondents were higher, with 39
  percent reporting always using oxygen when required above 25,000 feet
  and 48 percent above 41,000 feet.
  
  Part 135 respondents were nearer the Part 91 results, with 21 percent
  saying they always use supplemental oxygen when only one crewmember is
  at the controls above 25,000 feet and 18 percent continuously using
  oxygen when flying above 35,000 feet.


So for part 121 operators, the primary time when the regulation is relevant is when one pilot has left the cockpit above FL250. The article goes on to discuss some of the reasons pilots may not follow the regulation and whether the regulation makes good sense. Some pilots on the PPRuNe Forums discussed the rules here (before they were changed to FL410) and also here. One author notes, with regard to donning masks during bathroom breaks, that "there is probably no more violated FAR in US operations than this one." Several reasons are posited in those threads, among them:


Some non-military pilots do not receive decompression chamber training, and so do not have an appreciation for the effects of severe hypoxia and its dangers
This practice would use up much of the cockpit oxygen supply, especially on long-haul flights, leaving less available in the event of an emergency (some information on how long oxygen supplies tend to last)
The expense and time involved in topping off oxygen tanks all the time if they were routinely used. 
A desire to reduce the complexity of single-pilot operations during breaks, not wanting the remaining pilot to complicate things by donning the mask and adjusting audio settings accordingly instead of focusing on the aircraft
In some cases, restowing the masks may require maintenance personnel after the flight, which is impractical
The same rules do not apply under many other regulatory regimes, which leads people to conclude they are unimportant


Note that some pilots discuss using the masks routinely, both to ensure that their operations are as safe and legal as possible, and to give the crew practice and experience in donning and using the masks so they will be comfortable with them in the event of an emergency.