If they have to save Scalphunter and Arclight, they could at least keep them locked up, Scalphunter roaming the X-Mansion is just too dangerous. And to top it off, not only is he carrying weapons, he can form weapons out of his own body...don't they have containment cells in the mansion?

Gathering them at the mansion was allegedly a voluntary thing. They chose to go there, and therefore it must be treated as a haven. They knew the risks when they said that it would be a safe place for all mutants. To imprison people who voluntarily came to you would only undermine intermutant relationships, not to mention cause many additional problems that they can hardly handle right now. Imagine if those mutants then got freed and specifically as a result of that were upset and decided to take it out on all the rest of them. Additionally, I offer these quotes from the 198 to further cement this:
Valerie Cooper: This is NOT internment. This project is intended for the protection of mutants and operates on a strictly voluntary basis.
Kitty: So everyone's free to come and go?
Valerie Cooper: Not exactly, Kitty. We have some control.
Kitty: Concentration camp.
In a sense they are already imprisoned and being constantly guarded by giant sentinels provided by ONE. Members of the 198 who might attempt to cause problems have already been dealt with by use of force, even further damaging the trust of their shaky alliance. Another quote, just to further counter why it would be inappropriate to imprison specific mutants for past wrongs:
Valerie Cooper: Are you aware how many of the mutants in this camp have, at some time, been suspected of homicide?
Beast: All of them?
Its this kind of thinking of "lock up the dangerous mutant" that has lead to the rise of the Brotherhood, Magneto's war and even the House of M event and fallout. Several members of the X-Men have even been party to murder on several levels. Should they as well be locked up? To do any of this would be a clear violation of basic human rights, and while speaking in the context of the American based Xavier institute would be a deprivation of the due process rights provided by the constitution. And as for the carrying of weapons and the necessary restriction of those weapons, most of those mutants are living weapons, and to suggest that mutants should be locked up as a result of their weapon based powers reminds me of a scene from the first X-Men movie, the exchange between Jean Grey and Senator Kelly, as she reminded the Senator that the wrong driver behind the wheel of a car can be dangerous, and that while we liscence people to drive we can hardly liscence them to live. If scalphunter's ability to carry weapons (a right provided by the constitution as well, as might be noted) or produce weapons makes him a candidate for isolation and special monitoring, should not also Cyclops, head of this little operation, be set aside? After all, we saw what he could do to a sentinel in Astonishing X-Men 8. Wolverine and Beast also are natural weapons, and psylocke is without a doubt a very powerful weapon, not to mention the rest of the psychics. My point in all of this is that once you begin limitations of people based on their differences or due to a history of discrimination, things can get very iffy.