Pal Benko and other GMs withdrew from the 1970 Interzonal to allow Fischer to advance to the candidates. Although I do not claim to know which rules were in effect at this time, I always thought that a withdrawal is the same as a forfeiture, much like how Fischer lost his match with Reshevsky.
Did FIDE allow these GMs to cheat? If so, should FIDE strip Fischer of his title? Should FIDE have punished the other GMs?
NOTE: I am happy with this history, this is just questions to consider.
My thought it no, a withdrawal is not the same as a forfeiture until pairings are actually set, and possibly also if play begins in a match or event. In large events it does occur where a player cannot appear for whatever reason. That player’s slot is then filled by a different player. The largest most recent example that comes to mind is Magnus Carlsen’s 2010 withdrawl from the 2011 Candidates’ match / exit from the 2012 championship cycle. In my humble opinion the 2012 Championship should have been between Anand and Carlsen, not Gelfand. But I don’t think anyone would characterize Carlsen’s withdrawal as a “forfeiture,” except in the limited sense that he forfeited his own chance at the title.
The difference with Reshevsky 1961 is that the match had already been playing for 11 games. Fischer’s withdrawal was thus a forfeit.
As for ‘allowing to cheat’… I don’t know that I would characterize either the chess political maneuvering in general or the decision of an individual player to back out of a tournament with the “C” word. It is certainly less worse than the Soviet machinations of the time, if you believe certain sources. Even paranoids have enemies.
And no, talking of stripping Fischer’s title or other sanctions for events of 40 years ago? Ridiculous on its face. And I surely wouldn’t want to give FIDE yet more apparent power and control in order to sanction players today. It already has too much.
Oh come on… surely they’d never misuse this power. They only care about preserving the integrity and prestige of the game. 
I thought that each zone determined who would be sent to the interzonal, in whatever way that zone opted to do so. Most zones had a tournament to determine its entries. The US was it own zone, and used the US Championship to determine its players. Fischer did not play in the US championship due to some issues and it didn’t look like he wanted to play in the interzonal. After the US championship was over and the USCF was getting ready to send its list of participants to FIDE, it was determined that Fischer would play in the interzonal if he was named to that list. So the USCF contacted Pal Benko (the player who would have had the final zonal spot) and every player that scored less than Pal did to get their sign-off to submitting Fischer’s name as the last entry from the zone. All of them said that Fischer was the only person they would step aside for, but they would step aside for him.
This was very well known at the time and if Fischer had not had a legal submission of his name then you can be pretty sure that the Soviet bloc countries would have taken action at that time.
The Soviet players (who understood that Fischer was the only serious non-Soviet contender for the title) knew of USCF’s attempt to have Fischer play in the Interzonal, and they did not object: see Tal’s April 1970 comments in Russians vs. Fischer.
Of course, they had no idea how much stronger Fischer had become after his sabbatical.