The home side had a 37th minute Dylan McGowan goal ruled out after assistant referee Francis Andrews flagged for a foul by Jason Holt on United goalkeeper Radoslaw Cierzniak.
The Polish shot-stopper had appeared to misjudge Callum Paterson's long throw, leaving the Australian defender with a tap in.
Locke, whose team have now cut the gap at the Premiership basement to eight points on St Mirren, said: "I felt we should have taken all three points.
"I've seen the goal there on the laptop and it's a goal. It's another sore one to take because it's another big decision that's gone against us.
"The linesman disallowed the goal, I had a word at half-time and he said there was a push.
"Unfortunately we've just got to accept that and move on. From where I was it looked like the keeper just misjudged the throw.
"We've had a couple of bad decisions against us, we've got to get on with it. We can't let it affect us."
United had a goal disallowed of their own minutes earlier when Nadir Ciftci turned in a shot from close range.
But referee Steven McLean ruled that David Goodwillie had committed a foul in the build up.
United manager Jackie McNamara said: "I thought it was a free-kick (for Hearts' disallowed goal).
"I watched it back. I know it's soft but it's still a free-kick. The lads went into him first as he's thrown the ball and he's went in again to put him back towards to his goal.
"It's a foul. Nadir's one for the foul against Goody, I thought he just used his body there.
"I thought there was not much in that. I know Lockie is a bit frustrated with their goal but it was a foul."
United forward Ciftci went the closest during a captivating draw when he hit the post after the break.
Locke's disappointment over the main talking point was tempered by the return of Ryan Stevenson, who picked up a knee injury against Hibs on August 11.
Locke added: "I was delighted, he is a big player, a very important player for us. He is great in the dressing room as well so it's great to get him back on the pitch again."
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